Fungicide Resistance in Venturia effusa, Cause of Pecan Scab: Current Status and Practical Implications.

Pecan scab, caused by Venturia effusa, is the most economically damaging disease of pecan in the southeastern United States, and annual epidemics are most effectively managed through multiple fungicide applications. The fungicide applications can be the single greatest operating cost for commercial growers and the return on that investment is impacted by fungicide resistance. V. effusa produces multiple generations of conidia per season, exhibits substantial genetic diversity, overwinters as stromata in the tree, and is under immense selection from the applied fungicides, all of which lead to a high risk for developing fungicide resistance. Since the mid-1970s, resistance or reduced sensitivity has been observed in isolates of V. effusa to the methyl benzimidazole carbamates, demethylation inhibitors, quinone outside inhibitors, organotin compounds, and the guanidines. Over the last 10 years, several studies have been conducted that have improved both scab management and fungicide resistance management in V. effusa. The aim of this review is to summarize recent developments in our understanding of fungicide resistance in V. effusa in the context of scab management in southeastern pecan orchards. The history, modes of action, general use of the labeled fungicides, and mechanisms and stability of fungicide resistance in V. effusa are discussed; conclusions and future research priorities are also presented.

Spatial Variation and Temporal Dynamics of Fungicide Sensitivity in Venturia effusa Within a Pecan Orchard.

An 18-ha commercial pecan orchard was sampled over 3 years to study the spatial and temporal variation in fungicide sensitivity of Venturia effusa, cause of pecan scab. The orchard was divided into a two-dimensional, 8 × 8 grid of 64 quadrats, each containing nine trees (unless there were missing trees), and samples were collected once per year from each quadrat to be tested for sensitivity to fentin hydroxide, propiconazole, and thiophanate-methyl. Averaged across the orchard, insensitivity to all three fungicides was significantly lower in 2016 compared with 2015, but significantly greater for fentin hydroxide and thiophanate-methyl in 2017. Although significant spatial autocorrelation was observed for sensitivity to propiconazole in 2017 and for thiophanate-methyl in 2015 and 2017, indicating clustering, all other fungicide-by-year combinations were not significant. Omnidirectional spatial dependence was observed for sensitivity to propiconazole and thiophanate-methyl in 2017. In both instances, the semivariance increased linearly with lag distance; however, the range of spatial dependence was >276.5 m and could not be estimated accurately. Additionally, a separate sampling was conducted in all 3 years to identify an appropriate sampling size and pattern for fungicide sensitivity screening. A leaflet sample size of 165 in 11 groups of 15 allowed for accurate sensitivity testing for the three fungicides in all 3 years; however, a sample size of 45 leaflets in three groups of 15 was sufficient for quantifying sensitivity for propiconazole and thiophanate-methyl, in most cases. These results indicate that considerable biological variation in fungicide sensitivity exists in orchard-scale populations of V. effusa and that the spatial characteristics of those populations may differ in two-dimensional space depending on the growing season.

Disease and Yield Response of a Stem-rot-resistant and -Susceptible Peanut Cultivar under Varying Fungicide Inputs.

Peanut (Arachis hypogaea L.) producers rely on costly fungicide programs to manage stem rot, caused by Sclerotium rolfsii. Planting disease-resistant cultivars could increase profits by allowing for the deployment of less-expensive, lower-input fungicide programs. Field experiments were conducted to characterize stem rot and early and late leaf spot (caused by Passalora arachidicola and Nothopassalora personata, respectively), yield, and overall profitability of cultivars Georgia-06G (stem-rot-susceptible) and Georgia-12Y (stem-rot-resistant) as influenced by seven commercial fungicide programs. Stem rot incidence was consistently lower on Georgia-12Y for all fungicides when compared with Georgia-06G and was lowest for both cultivars in plots treated with prothioconazole plus a tank mixture of penthiopyrad and tebuconazole. Leaf spot severity was similar for both the resistant and susceptible cultivars, and the greatest reduction occurred in plots treated with prothioconazole plus a tank mixture of penthiopyrad and tebuconazole. Fungicide programs gave similar yield and net return on Georgia-12Y; however, plots of Georgia-06G treated with prothioconazole plus a tank mixture of penthiopyrad and tebuconazole had the greatest yield and net return. Yields and economic return from the highest level of fungicide inputs on Georgia-06G were numerically less than those of Georgia-12Y treated with only chlorothalonil. These results show the value of fungicides in peanut disease management with susceptible cultivars, as well as the benefits of planting stem-rot-resistant cultivars in high-risk situations.

Dynamics of Fungicide Sensitivity in Venturia effusa and Fungicide Efficacy under Field Conditions.

Venturia effusa, which causes pecan scab, has developed resistance to fungicides that were once effective. Over 2 years, laboratory-based sensitivity of fentin hydroxide (TPTH) and tebuconazole in V. effusa and their efficacy under field conditions were compared. Leaf and nut scab were assessed on pecan trees receiving 10 applications of TPTH, tebuconazole, azoxystrobin, azoxystrobin plus tebuconazole, TPTH plus tebuconazole, or no fungicide (NTC) per year. Sensitivity of V. effusa on leaflets collected from treated and nontreated trees was assessed in June and September, respectively. The mean relative germination (RGe) on TPTH at 30 µg/ml was 10.9 and 40.9% in 2016 and 4.2 and 0.6% in 2017. Mean relative growth (RGr) on tebuconazole at 1 µg/ml was 45.5 and 34.6% in 2016 and 69.3 and 56.3% in 2017. In both years, leaf and nut scab were significantly lower on trees treated with azoxystrobin, azoxystrobin + tebuconazole, or TPTH + tebuconazole when compared with NTC and tebuconazole-treated trees. Compared with the NTC, tebuconazole did not significantly reduce leaf scab in 2017 or nut scab in either year, indicating that an RGr value between 34.6 and 69.3% is likely to result in a control failure on tebuconazole-treated trees. Although better activity was expected, TPTH reduced scab with RGe values between 0.6 and 40.9%. These results are valuable for developing fungicide sensitivity thresholds to better predict fungicide performance.

Location of an Intron in the Cytochrome b Gene Indicates Reduced Risk of QoI Fungicide Resistance in Fusicladium effusum.

Pecan scab, caused by Fusicladium effusum, is most effectively managed using multiple fungicide applications, including quinone outside inhibitors (QoIs). However, QoIs have a high risk for resistance developing in phytopathogenic fungi. QoI resistance is generally associated with amino-acid substitutions at positions 129, 137, and 143 of the cytochrome b (cytb) gene. A substitution at position 143 confers complete resistance, while an intron immediately downstream of this position prevents the substitution. The objective of this study was to assess the risk of QoI resistance by characterizing a partial fragment of the F. effusum cytb gene. Sequence analysis of the 1,919-bp fragment revealed the presence of a 1,407-bp intron immediately downstream of position 143. This intron was identified in 125 isolates collected from 16 counties across the state of Georgia. No substitutions were identified at positions 129 or 143 but, in seven of the isolates, glycine was replaced with serine at position 137. The ubiquitous nature of the detected intron provided strong evidence that the G143A substitution may not occur in F. effusum isolates, although resistance could still develop through intron loss events or the selection of intron-lacking genotypes, or as the result of other mutations in the cytb gene.

Management of Daylily Rust with Different Fungicide Combinations and Spray Intervals.

Daylily (Hemerocallis spp.) is a popular herbaceous perennial plant and was considered to be relatively disease free until 2000, when daylily rust, caused by Puccinia hemerocallidis, was first detected in the United States. Management of daylily rust in nurseries is dependent on the use of fungicides, which are typically applied to the foliage of large blocks of plants at 21- or 28-day intervals. The objectives of this study were to determine the most effective fungicides or fungicide combinations and application intervals for managing daylily rust in the field. Foliar sprays of azoxystrobin alone at 14-, 21-, or 28-day intervals, combinations of azoxystrobin + propiconazole, azoxystrobin + chlorothalonil, propiconazole + chlorothalonil, and chlorothalonil + thiophanate-methyl applied at intervals of 21or 28 days, and a nontreated control were evaluated under high disease pressure, at three locations in Griffin, GA in 2014. In all three fields, all treatments that included azoxystrobin were effective at reducing area under the disease progress curve (AUDPC) compared with the nontreated control. At two of the three locations, azoxystrobin applied at 14-day intervals had significantly lower AUDPC than when applied at 21- or 28-day intervals. The addition of propiconazole or chlorothalonil to azoxystrobin did not improve rust control. Disease ratings for propiconazole + chlorothalonil and thiophanate-methyl + chlorothalonil applied at 21- or 28-day intervals did not differ from the untreated control. The 21-day treatments resulted in significantly lower disease than 28-day treatments (all fungicides) in the middle and end of the season. Elimination of less efficacious active ingredients and unnecessary applications can help growers maximize profitability by reducing expenses as well as simplifying fungicide inventory and storage.

Assessing Systemicity of Peanut Fungicides Through Bioassay of Plant Tissues with Sclerotium rolfsii.

To better understand movement of systemic fungicides in peanut (Arachis hypogaea), three terminal, fully expanded leaves of primary lateral branches of 'Tifrunner' peanut were treated with prothioconazole + tebuconazole (Provost, 0.29 kg a.i./ha), azoxystrobin (Abound, 0.31 kg a.i./ha), or flutolanil (Moncut, 0.79 kg a.i./ha) in field experiments. Basipetal leaves and pods on the same branch with the treated leaves were sequentially numbered from 1 to 3, with 1 being closest to treated foliage. These nontreated tissues, with newly formed terminal leaves, were sampled 4, 8, and 12 days after treatment for bioassay with Sclerotium rolfsii. All fungicides protected new acropetal leaves while prothioconazole + tebuconazole also provided some inhibition of S. rolfsii in nontreated basipetal leaves but no fungicide protected pods. In the greenhouse, applications of prothioconazole + tebuconazole or prothioconazole (Proline, 0.18 kg a.i./ha) to main stems of 'Georgia Green' provided some protection to leaves from nontreated cotyledonary branches sampled 14 days after last treatment but S. rolfsii was not inhibited on nontreated roots, stems, or pods. The results demonstrate acropetal protection by all fungicides evaluated, and indicate that prothioconazole + tebuconazole or prothioconazole applied to foliage can sometimes reduce diseases in the lower, nontreated portions of the plant.

Irrigation Timing Impacts the Efficacy of Foliar-Applied Fungicides Toward Foliar and Soilborne Pathogens of Peanut.

Fungicides not reaching target organisms result in decreased disease control. In the southeastern United States, foliar-applied fungicides are routinely used to manage peanut (Arachis hypogaea) diseases. Irrigation is often applied to wash fungicides from treated foliage to obtain maximum control of diseases caused by soilborne pathogens. Administering irrigation before fungicide residues have dried may adversely impact foliar disease control. A microplot study was conducted in 2003, 2004, and 2005 to evaluate the redistribution of azoxystrobin, tebuconazole, and flutolanil plus chlorothalonil following different irrigation timings. Standard fungicide regimes were subjected to 1.3-cm of irrigation 0, 6, 12, 24, 48, or 96 h after application, and a nonirrigated control was included. Microplots not receiving irrigation were covered while irrigation treatments were administered. Irrigation timing was significant for the number of early leaf spot (Cercospora arachidicola) lesions per leaf. Leaf spot was more severe when irrigation was administered immediately following fungicide applications, and was significantly reduced with a 6- and 12-h delay prior to an irrigation event, whereas maximum control was obtained when irrigation was delayed for 24 h or later. To further quantify fungicide residue distribution, Sclerotium rolfsii was used to bioassay foliage and pods. Lesion development on leaflets, which was greater for earlier irrigation timings, did not differ for the 12-h and later timings and was generally similar to the nonirrigated controls. Pod colonization for all fungicides increased according to a quadratic function of irrigation timing, with the least colonization occurring at the 0-h timing. Colonization of pods treated with azoxystrobin was similar for all irrigation timings; whereas, suppression was greatest for tebuconazole at earlier irrigation timings. This study demonstrates that irrigation can be used to redistribute fungicides applied to peanut foliage to improve control of soilborne pathogens but administering irrigation within 24 h may decrease leaf spot control.

Interactive Effects of Planting Date and Cultivar on Tomato Spotted Wilt of Peanut.

Field experiments were conducted at Gainesville and Marianna, FL in 2004 and 2005 in which severity of spotted wilt, caused by Tomato spotted wilt virus, and pod yield were compared in six peanut (Arachis hypogaea) cultivars. The six cultivars included the moderately field resistant cultivars ANorden, C-99R, and Georgia Green; the highly field resistant cultivars AP-3 and DP-1; and the susceptible cultivar SunOleic 97R. There were four trials at each location, with four planting dates that ranged from late March to early June. Tomato spotted wilt severity in moderately resistant and susceptible cultivars was lower at Gainesville than at Marianna in both years in moderately resistant and susceptible cultivars. Trends in incidence for the two locations were less evident for AP-3 and DP-1. At Gainesville, there were few differences in tomato spotted wilt severity, and severity ratings were similar for Georgia Green and SunOleic 97R in two of four trials in 2004 and across all trials in 2005. At Marianna, severity ratings were lower for Georgia Green than for SunOleic 97R in six of the eight trials, and severity of tomato spotted wilt was lower for AP-3, C-99R, and DP-1 than for Georgia Green in all eight trials. In 2004, there was a trend toward decreasing severity ratings for Georgia Green and SunOleic 97R with later planting dates, but not for AP-3 or DP-1 at Marianna. Split-plot field experiments were also conducted at Tifton, GA in 2005 through 2007 in which incidence of tomato spotted wilt and pod yield were compared for peanut cultivars AP-3 and Georgia Green across planting dates ranging from late April through late May. Incidence of tomato spotted wilt was lower for AP-3 than for Georgia Green within each planting date of all years, and planting date effects were smaller in AP-3, if observed at all, than in Georgia Green. In most planting dates of all three trials, yields were higher for AP-3 than for Georgia Green. The relationships between yield and planting date were not consistent. These results indicate that the level of field resistance in AP-3 and DP-1 cultivars is sufficient to allow planting in late April without greatly increasing the risk of losses to tomato spotted wilt.

Night Spraying Peanut Fungicides II. Application Timings and Spray Deposition in the Lower Canopy.

Chemical control of soilborne peanut (Arachis hypogaea) diseases requires deposition of fungicide on plant tissues near the soil. Four applications of a protectant fungicide, chlorothalonil (1.26 kg a.i./ha), or a systemic, azoxystrobin (0.21 kg a.i./ha), pyraclostrobin (0.21 kg a.i./ha), or prothioconazole (0.08 kg a.i./ha) plus tebuconazole (0.15 kg a.i./ha), were sprayed either (i) early in the morning (3:00 to 5:00 A.M., with folded and wet leaves), (ii) during daylight (10:00 A.M. to 12:00 P.M., with unfolded and dry leaves), or (iii) in the evening (9:00 to 10:00 P.M., with folded and dry leaves). All timings of systemic fungicides provided similar control of foliar diseases. Early-morning applications of pyraclostrobin and prothioconazole plus tebuconazole decreased stem rot (caused by Sclerotium rolfsii) at digging compared with day and evening applications. All systemic fungicides increased yield when applied at early-morning compared with day applications. Spray coverage, density, and droplet size were higher with night than day applications, and differences were more evident in the lower canopy layers. These results suggest that applications made early in the morning to folded, wet leaves can improve spray penetration of peanut canopies, thus improving stem rot control and increasing yield.

Night Spraying Peanut Fungicides I. Extended Fungicide Residual and Integrated Disease Management.

The efficacy of chemical control of stem rot (caused by Sclerotium rolfsii) of peanut (Arachis hypogaea) relies partially on increasing deposition and residual activity in the lower canopy. Tebuconazole (0.21 kg a.i./ha, four applications) and azoxystrobin (0.31 kg a.i./ha, two applications) were each applied on peanut plants in daylight or at night, when leaves were folded, in two Tifton, GA, field trials in 2007. Both timings of each fungicide provided similar control of early leaf spot (caused by Cercospora arachidicola). Night applications of azoxystrobin and tebuconazole reduced stem rot at digging and increased yield compared with day applications. Night applications of tebuconazole were also tested in Nicaragua from 2005 to 2007. Peanut plants had less stem rot, similar levels of rust (caused by Puccinia arachidis), and higher yield with night applications than with day applications. Residual activity of azoxystrobin and tebuconazole were improved on the bottom shaded leaves (on which fungicides would be better deposited with night application) compared with top, sun-exposed leaves (where most fungicide would be deposited with a day application) according to a bioassay with S. rolfsii. Increased fungicide residual activity within the bottom canopy may increase fungicide efficacy on stem rot and augment peanut yield.

Resistance in Peanut Cultivars and Breeding Lines to Three Root-Knot Nematode Species.

Three major species of root-knot nematode infect peanut: Meloidogyne arenaria race 1, M. hapla, and M. javanica race 3. Sources of resistance to all three nematodes are needed for developing novel peanut cultivars with broad resistance to Meloidogyne spp. Cultivars and breeding lines of peanut were evaluated for resistance to M. arenaria, M. hapla, and M. javanica in the greenhouse and in the laboratory. Twenty-six genotypes with some resistance to M. arenaria, M. javanica, or M. hapla were identified from 60 accessions based on average eggs per gram of root and gall index relative to a susceptible control. Among these, 14 genotypes were moderately to highly resistant to all three species, 5 genotypes were resistant to M. arenaria and M. javanica, 2 genotypes were resistant to M. javanica and M. hapla, 1 genotype was resistant M. arenaria alone, and 4 genotypes were resistant to M. hapla alone. Reproduction of M. arenaria on lines NR 0817, C724-19-11, and D108 was highly variable, indicating that these genotypes likely were heterogeneous for resistance. COAN, NemaTAM, C724-25-8, and the M. arenaria-resistant plants of C724-19-11 contained the dominant sequence-characterized amplified region marker (197/909) for nematode resistance. Results with the molecular markers indicate that the high resistance to M. arenaria in GP-NC WS 6 may be different from the resistance in COAN, NemaTAM, and C724-25-8. Resistance to M. arenaria was correlated with resistance to M. javanica in peanut, whereas resistance to M. hapla was not correlated with the resistance to either M. arenaria or M. javanica. The resistant selections should be valuable sources for pyramiding resistance genes to develop new cultivars with broad and durable resistance to Meloidogyne spp.

Evaluation of Weather-Based Spray Advisories for Improved Control of Peanut Stem Rot.

Stem rot of peanut, caused by the soilborne fungus Sclerotium rolfsii, is greatly influenced by environmental conditions. Disease management programs rely heavily on fungicides, which are applied on a calendar-based program. To determine whether improved control of stem rot could result from weather-based spray advisories, models were constructed using what is currently known about the biology of S. rolfsii and etiology of stem rot epidemics in peanut. Spray advisories based on soil temperature, precipitation, and host parameters were tested, along with advisories focusing on soil temperature and precipitation or precipitation alone. The advisories were evaluated and compared with the currently used calendar-based program over four locations annually for 3 years. Fungicide application timing had a significant effect on both stem rot control and resulting pod yields. In general, stem rot control following the advisories considering soil temperature, precipitation, and canopy growth was similar or better than that offered by the calendar-based program, but yields generally were comparable. The AU-Pnut advisory for foliar diseases also was effective for scheduling azoxystrobin applications for stem rot.

Use of Resistant Cultivars and Reduced Fungicide Programs to Manage Peanut Diseases in Irrigated and Nonirrigated Fields.

Field experiments were conducted in 2004 and 2005 to evaluate the response of several peanut cultivars to standard and reduced-input fungicide programs under production systems which differed in the duration of crop rotation, disease history within a field, or in the presence or absence of irrigation. Effects on early leaf spot (caused by Cercospora arachidicola), late leaf spot (caused by Cercosporidium personatum), and southern stem rot (caused by Sclerotium rolfsii), pod yields, and economic returns were assessed. Standard fungicide programs were similar for both sets of experiments and included applications of pyraclostrobin, tebuconazole, azoxystrobin, or chlorothalonil. Reduced-fungicide programs, comprising combinations of the aforementioned fungicides, resulted in two and four applications for the cultivar and irrigation experiment, respectively. Two additional programs (a seven-spray chlorothalonil and a nontreated control) were included in the cultivar experiment. Fungicide programs provided adequate levels of leaf spot suppression, and stem rot incidence was similar among fungicide programs within the two management systems. In the cultivar experiment, returns were significantly lower for the reduced program compared with the full program and seven-spray chlorothalonil program; however, they were significantly higher than the nontreated control. Significant differences in leaf spot, stem rot, and yield were observed among cultivars in both experiments. Overall, leaf spot intensity was lowest for the cvs. Georgia-03L and Georgia-01R and greatest for Georgia Green and Georgia-02C. Georgia-03L, Georgia-02C, and AP-3 consistently had lower incidence of stem rot than the other cultivars. Pod yields for all cultivars were equivalent to or greater than Georgia Green in both experiments; however, the performance of reduced-fungicide programs was inconsistent.

Effects of Row Pattern, Seeding Rate, and Inoculation Date on Fungicide Efficacy and Development of Peanut Stem Rot.

Two field studies were conducted in 2000, 2001, and 2002 to determine the effects of row pattern (91.4-cm single or 20.3-cm twin) and seeding rate (single: 12.5, 17.4, or 22.6 seed m-1or twin: 6.2, 8.9, or 11.5 seed m-1) on peanut stem rot (Sclerotium rolfsii) development. The first study was conducted in a naturally infested field and relative efficacy of azoxystrobin (Abound 2.08 F, applied at a rate of 0.3 kg a.i. ha-1 at 60 and 90 days after planting [DAP]) also was evaluated. In this study, stem rot incidence was significantly greater (P < 0.05) in single rows planted at high seeding rates than in twin rows planted at any of the seeding rates. Row pattern did not affect azoxystrobin efficacy, and disease incidence was nearly half as much in twin rows treated with fungicide than incidence in single rows treated with fungicide. In the second field study, individual peanut plants in fumigated plots were inoculated once with S. rolfsii at 50, 70, or 90 DAP. Stem rot incidence at harvest was significantly greater on plants inoculated 50 DAP than plants inoculated 70 or 90 DAP. The incidence of spread to adjacent rows was higher in plots where plants were inoculated at 50 than at 90 DAP. Plants inoculated 90 DAP had less disease at harvest, but often developed more severe symptoms within the first week after inoculation compared with plants inoculated 50 or 70 DAP. Symptoms were more severe in single than in twin rows, and at the higher seeding rates. Data from these studies suggest that the physical spacing between plants is a critical factor in stem rot development both on individual plants and in plant populations.

First Report of Sclerotinia Blight Caused by Sclerotinia sclerotiorum on Peanut in Georgia.

Sclerotinia blight is one of the most economically important diseases of peanut (Arachis hypogaea L.) in Oklahoma and Virginia. Yield losses of 10% are common in these areas; however, losses may exceed 50% in highly infested fields (1). While Sclerotinia minor is considered the primary causal agent, S. sclerotiorum may also incite the disease. Symptoms typically appear late in the season and are favored by cool temperatures and high relative humidity (RH). Initial symptoms include wilting and yellowing of main or lateral branches. Dense mats of white mycelium develop on diseased areas, and small water-soaked lesions are apparent near the soil line. Lesions become bleached and infected tissues have a shredded appearance. Sclerotia are produced on and inside infected plant parts (2). During October 2004, following a period of heavy rainfall and cool temperatures, peanut plants (cv. Tifrunner) with these symptoms were observed in a field near Surrency, GA. The field had been planted to cotton (Gossypium hirsutum L.) for many years and peanut was strip-tilled into a heavy rye (Secale cereale L.) cover. Disease foci were found throughout the field and final incidence was 20%. Stem sections were surface disinfested in 0.5% sodium hypochlorite for 1 min and plated on potato dextrose agar (PDA). Cultures of S. sclerotiorum (2) were recovered after incubation at 20°C for 2 weeks. Pathogenicity tests were conducted by inoculating wounded peanut mainstems with PDA plugs either with or without the fungus. Inoculation sites were wrapped with moistened cheesecloth, and plants were incubated in a dew chamber at 20°C and 95% RH. There were a total of four replications and the experiment was repeated once. Symptoms consistent with those observed in the field appeared after 3 days and lesion lengths were measured after 5 days. Average lesion lengths were 1.4 and 1.6 cm for cvs. Georgia Green and Tifrunner, respectively Controls remained symptomless. Sections of symptomatic tissue were plated on PDA, and S. sclerotiorum was reisolated from 100% of symptomatic tissue. Although S. sclerotiorum is a common pathogen of various winter crops and weeds found in the southeast, to our knowledge, this is a first report of Sclerotinia blight on peanut in the region. No other occurrences of the disease have been reported since the initial discovery; however, potential losses could be incurred if peanuts are planted in infested fields and harvest is delayed. References: (1) H. A. Melouk and P. A. Backman. Management of soilborne fungal pathogens. Pages 75-85 in: Peanut Health Management. H. A. Melouk and F. M. Shokes, eds. The American Phytopathologicial Society, St. Paul, MN, 1995. (2) D. M. Porter and H. A. Melouk. Sclerotinia blight. Pages 34-36 in: Compendium of Peanut Diseases. 2nd ed. N. Kokalis-Burelle et al., eds. The American Phytopathologicial Society, St. Paul, MN, 1997.

Integrated Disease Management of Leaf Spot and Spotted Wilt of Peanut.

Field experiments were carried out to evaluate the effects of integrated management of early leaf spot, caused by Cercospora arachidicola, and spotted wilt, caused by Tomato spotted wilt virus (TSWV), on peanut (Arachis hypogaea) using host resistance, two tillage systems, and varying fungicide programs. Effects on pod yield and economic return were assessed. Genotypes C-11-2-39 and Tifrunner demonstrated the best field resistance to TSWV, whereas cvs. DP-1 and GA-01R and line C-28-305 were among the genotypes with the best leaf spot resistance. Epidemics of both diseases were comparable or suppressed in strip-tilled plots compared with conventionally tilled plots. Leaf spot intensity decreased with increased fungicide applications, but to a lesser degree with use of resistance and strip tillage. Yields and net returns were similar between tillage treatments in 2002 and lower in strip tillage in 2003. Genotypes with the greatest yields and returns were C-11-2-39, C-99R, and GA-01R. Returns were comparable among the four-, five-, and seven-spray programs in both years, despite differences in yield. The standard production system, Georgia Green in conventional tillage with seven sprays, resulted in lower returns than half the integrated systems tested in 2002, but had comparable or higher returns than nearly all systems in 2003. When significant, yields and returns were correlated with spotted wilt intensity to a greater degree than leaf spot intensity.

First Report of Botrytis Blight of Peanut Caused by Botrytis cinerea in Georgia.

Because of the importance of spotted wilt caused by Tomato spotted wilt virus (TSWV), most peanut (Arachis hypogaea L.) breeding programs in the southeastern United States are focusing on developing resistance to TSWV. Many of the cultivars with improved resistance to TSWV are late maturing, requiring 150 days to reach optimum maturity. This factor could greatly impact disease problems at harvest. During November of 2004, an unknown disease was observed on peanut cvs. Georgia 02-C and Hull in a commercial field in Appling County. Symptoms included wilting stems with water-soaked lesions and a dense, gray mold growing on infected tissues. Final disease incidence was less than 5%. For isolation, diseased tissue was surface sterilized by soaking in 0.5% sodium hypochlorite for 1 min, air dried, plated on potato dextrose agar (PDA), and incubated at 20°C. Botrytis cinerea Pers.:Fr., causal agent of Botrytis blight, was isolated from the margins of infected tissue. Mycelia were initially white but became gray after 72 h at which time tall, branched, septate conidiophores formed. Mature, unicellular, ellipsoid, hyaline conidia (8.9 × 10.4 µm) formed in botryose heads (1). Hard, black, irregular-shaped sclerotia formed after 2 weeks. Stems of greenhouse-grown peanut plants (cv. Georgia Green) were inoculated with PDA plugs colonized with either B. cinerea or B. allii Munn. Inoculations were made 3 cm below the last fully expanded leaf on wounded and nonwounded tissue. Noncolonized PDA plugs served as controls (n = 9). Plants were arranged in a dew chamber at 20°C in a randomized complete block design. Lesions and spore masses identical to those observed in the field appeared 3 to 5 days after being inoculated with B. cinerea. The B. allii inoculations caused only superficial lesions. After 5 days, mean lesion lengths for B. cinerea were 59 and 37 mm for wounded and nonwounded inoculations, respectively. B. cinerea was recovered from 100% of the symptomatic tissues. Botrytis blight is considered a late-season disease that occurs in cool, wet weather (3). Symptoms similar to those of Botrytis blight were observed on mature and over-mature peanut in Georgia and have been cited as "unpublished observations" (2); however, to our knowledge, this is the first report of the disease in Georgia. Although Botrytis blight is not considered a major peanut disease, it may become more prevalent at harvest as producers utilize late-maturing cultivars to manage spotted wilt. References: (1) H. L. Barnett and B. B. Hunter. Illustrated Guide of Imperfect Fungi. 4th ed. The American Phytopathological Society, St. Paul, MN, 1998. (2) K. H. Garren and C. Wilson. Peanut Diseases. Pages 262-333 in: The Peanut, the Unpredictable Legume. The National Fertilizer Assoc. Washington D.C. 1951. (3) D. M. Porter. Botrytis blight. Pages 10-11 in: Compendium of Peanut Diseases. 2nd ed. N. Kokalis-Burelle et al., eds. The American Phytopathological Society, St. Paul, MN. 1997.

Effects of Plant Spacing, Inoculation Date, and Peanut Cultivar on Epidemics of Peanut Stem Rot and Tomato Spotted Wilt.

Two microplot studies were conducted with peanut (Arachis hypogaea L.) in 2000, 2001, and 2002 to determine the effects of plant spacing, inoculation date, and cultivar on stem rot development caused by Sclerotium rolfsii Sacc., tomato spotted wilt incidence, and microclimate (temperature and relative humidity). Stem rot severity and incidence decreased as plant spacings were increased in 5-cm increments from 5 to 30 cm. Two cultivars with similar susceptibility but different growth habits were compared. Perhaps due to heavy irrigation and extensive vegetative growth, stem rot was similar for 'Florida MDR-98' and 'Georgia Browne'. Plants inoculated later in the year (90 days after planting [DAP]) had less disease at harvest, but often developed more severe symptoms within the first week of inoculation than plants inoculated at 50 or 70 DAP. Canopy microclimate was different than ambient conditions for all treatments; however, differences among treatments were inconsistent and did not explain differences in disease among spacings. Generally, as plant spacing decreased and plant population increased, stem rot increased and tomato spotted wilt was reduced. These data demonstrate that the physical spacing between plants is a critical factor in disease development.

First Demonstration of Koch's Postulates for Lasiodiplodia theobromae Fruit Spot on Eggplant (Solanum melongena).

During October 2004, diseased eggplant fruit from a commercial farm in Colquitt County, Georgia, developed circular, tan, water-soaked lesions. Gray, septate mycelia quickly covered the fruit. Diseased fruit became shriveled, spongy, and mummified. Disease incidence in the field was approximately 1%. Lasiodiplodia theobromae (Pat.) Griffon & Maubl. (synonym Botryodiplodia theobromae Pat.) (2) was isolated from the margins of lesions and cultured on acidified potato dextrose agar. The fungus produced grayish colonies with aerial hyphae and black ostiolate pycnidia massed into stroma. Mature elliptical conidia (25.8 × 15.6 µm) were brown, had a single septation, and longitudinal striations. Isolates obtained from peanut and pecan were included in the pathogenicity tests. Mature fruit cv. Nightshade were surface disinfested for 30 s in 70% ethanol, followed by 60 s in 0.5% sodium hypochlorite, rinsed twice in sterile distilled water, and allowed to dry. Inoculations were made by placing an agar plug containing L. theobromae mycelial side down on the surface of the fruit or wounding with a sterile toothpick containing mycelium of the fungus. Fruit similarly inoculated with agar plugs or sterile toothpicks served as controls. There were a total of three replicates. Fruit were placed in plastic containers lined with moistened paper towels. Containers were placed in a dew chamber and incubated (28°C, relative humidity >95%) for 3 days, and then evaluated. Symptoms identical to those observed on naturally infected fruit developed on inoculated fruit. Controls remained disease free. L. theobromae was reisolated from all symptomatic tissue, satisfying Koch's postulates. Disease damage on wounded fruit was twice that of nonwounded fruit. However, seven of nine inoculations with agar plugs containing L. theobromae resulted in infection. Lesion lengths from wound inoculations were 9.8, 7.3, and 5.2 cm for isolates from peanut, pecan, and eggplant, respectively. Generally, L. theobromae is considered a facultative wound pathogen or a secondary invader (3). However, this study suggests that direct infection can occur. Although fruit spot has been reported previously on eggplant (1), to our knowledge, this is the first report verifying L. theobromae as the causal agent. References: (1) S. A. Alfieri et al. Index of Plant Diseases in Florida. Fla. Dep. Agric. Consum. Serv. Bull. 11, 1984. (2) H. L. Barnett and B. B. Hunter. Illustrated Guide of Imperfect Fungi. 4th ed. The American Phytopathological Society St. Paul, MN, 1998. (3) P. M. Phipps and D. M. Porter. Plant Dis. 82:1205, 1998.