Effects of Silicon and Fungicides on the Control of Leaf and Neck Blast in Upland Rice.

Silicon (Si) has been shown to suppress diseases of rice such as blast when applied to Si-deficient soils. In 1995 and 1996, Si was applied at 1,000 kg ha-1 to at two locations in eastern Colombia to determine if Si combined with reduced rates of fungicide could be used to manage leaf and neck blast effectively in upland rice. Two applications of edifenphos and three applications of tricyclazole were made at 0, 10, 25, or 100% of their labeled rates after amendment of soil with Si. At both locations, Si alone and Si combined with edifenphos reduced leaf blast severity by 22 to 75% when compared with nonamended, untreated controls, and suppression of leaf blast by Si alone was equal to or better than the full rate of edifenphos alone. Si alone suppressed neck blast as effectively or better than the full rate tricyclazole when severity was low; however, when severity was higher, a 10% rate of tricyclazole was needed in addition to Si. No differences in yield were observed between Si without fungicides and fungicides (full rate) without Si. Applications of Si made in 1995 had significant residual activity in terms of disease control and yield in 1996. The application of Si to Si-deficient soils may permit the use of reduced rates of fungicide to manage leaf and neck blast in upland rice.

Influence of Silicon and Chlorothalonil on the Suppression of Gray Leaf Spot and Increase Plant Growth in St. Augustinegrass.

Silicon, applied as calcium silicate slag (20% Si), was evaluated for its potential to suppress gray leaf spot (Magnaporthe grisea) and increase plant growth in newly planted St. Augustinegrass in the summers of 2000 and 2001. Calcium silicate was applied (1,000 kg Si/ha) to three sites that contained Si-deficient Histosols prior to sprigging St. Augustinegrass, cv. Floratam, in southern Florida. This treatment was compared with foliar sprays of chlorothalonil, calcium silicate plus chlorothalonil, and an untreated control. Immediately after planting, applications of chlorothalonil (720 g/liter) were made every 10 days for a total of seven sprays at a rate of 7.6 kg a.i./ha with a commercial sprayer. Based on area under the disease progress curve (AUDPC) values for the treatments calcium silicate alone, chlorothalonil, and calcium silicate plus chlorothalonil, gray leaf spot was reduced by 7, 65, and 68% at site one, 28, 34, and 59% at site two, and 41, 55, and 68% at site three, respectively when compared with the untreated control. The application of calcium silicate alone significantly reduced the final AUDPC of gray leaf spot when compared with the control at site two only. However, when disease severities were analyzed by week, the calcium silicate treatment significantly reduced the percentage of disease at weeks 2, 6, and 8 at site one and weeks 3 to 8 at site three when compared with the control. The final percent bare ground coverage for St. Augustinegrass was increased significantly using calcium silicate by 17 and 34% over the control at sites one and two, respectively (P ≤ 0.05). Silicon was the only element to significantly increase in the leaf tissue for treatments amended with calcium silicate. Levels of Si in leaves for treatments amended with calcium silicate were from 1.2 to 1.3%, while those not receiving calcium silicate had only 0.6 to 0.7%. Amendments with calcium silicate slag for St. Augustinegrass sod production on Si-deficient soils may be an option to reduce gray leaf spot development in newly sprigged fields and promote earlier ground coverage of grass when the environment is favorable for disease.

Outbreak of Choanephora Blight Caused by Choanephora cucurbitarum on Green Bean and Pepper in Florida.

Severe outbreaks of Choanephora blight on green bean (Phaseolus vulgaris cvs. Bronco, Shade, and Gold Mine) and bell pepper (Capsicum annuum cvs. Aristotle, Crusader, and Sentry) were widespread in southwestern (Hendry and Collier counties) and northern Florida (Alachua County) in October and November 2002. Disease incidence, estimated by inspecting 100 randomly selected bean plants in each of four fields, was 40 to 100% and infected fruit ranged from less than 10 to 100%. Incidence estimated similarly on pepper plants in three fields was 35 to 40% with substantial fruit infection observed predominantly around the calyx. Zucchini fruit and a pigweed plant (Amaranthus sp.) were observed with sporulating lesions of Choanephora, indicating that other hosts were affected during the outbreak. Symptoms were blighted leaves, dieback of shoot tips, blighted blossoms, and black, soft-rot lesions on fruit. Choanephora sp. was sporulating in abundance on diseased tissue. Isolates of Choanephora sp. grew readily as pure cultures on acidified potato dextrose agar and malt yeast extract (MYE) agar. C. cucurbitarum (Berk. & Rav.) Thaxter was identified on the basis of shape and ornamentation of the sporangiola (1). The sporangiola of C. cucurbitarum are ellipsoid to broadly ellipsoid, and the wall is usually longitudinally striate. Pathogencity tests consisted of spray inoculation (5,000 spores per ml) of five 6-week-old plants each with and without wounding made by lightly scratching the leaf surface with a needle. Plants were placed in the greenhouse with temperatures ranging from 21 to 26°C, and symptom development was observed as early as 3 days after inoculation. The percentage of infected plants after wounding was 40% for bell pepper ('Enterprise'), 100% for green bean ('Opus'), 0% for watermelon (Citrullus lanatus 'Star Gazer'), 60% for cantaloupe (Cucumis melo 'Vienna'), and 20% for cucumber (Cucumis sativus 'Thunder CY'). Lesions on inoculated leaves were similar to those seen in the field on bean and pepper, and sporulation of C. cucurbitarum was present in the necrotic areas on all symptomatic plants. Pure cultures of C. cucurbitarum were reisolated. C. curcurbitarum was observed and isolated from a few noninoculated bean flowers and two noninoculated bean pods indicating spread to noninoculated plants; otherwise control plants were asymptomatic. Unwounded plants did not develop lesions, indicating that wounding was necessary for infection by this inoculation technique. The mating type was determined by juxtaposing several isolates on MYE agar, and zygospore formation was observed indicating both + and - strains occur in Florida. These outbreaks show that under the proper environmental conditions, such as long periods of high rainfall, high humidity, and high temperatures, crops like bean and pepper that are not usually affected by the disease may experience significant damage. Reference: (1) P. M. Kirk. Mycol. Pap. 152:1-61, 1984.

The influence of silicon on components of resistance to blast in susceptible, partially resistant, and resistant cultivars of rice.

ABSTRACT The application of silicon (Si) fertilizers reduces the severity of blast, caused by Magnaporthe grisea, in irrigated and upland rice; however, little research has been conducted to examine the epidemiological and etiological components of this reduction. Four cultivars of rice with differential susceptibilities to race IB-49 of M. grisea were fertilized with three rates of a calcium silicate fertilizer and inoculated with the pathogen to test the effects of Si on the following components of resistance to blast: incubation period, latent period, infection efficiency, lesion size, rate of lesion expansion, sporulation per lesion, and diseased leaf area. For each cultivar, the incubation period was lengthened by increased rates of Si, and the numbers of sporulating lesions, lesion size, rate of lesion expansion, diseased leaf area, and number of spores per lesion were reduced. Lesion size and sporulation per lesion were lowered by 30 to 45%, and the number of sporulating lesions per leaf and diseased leaf area were significantly reduced at the highest rate of Si. The net effect of Si on these components of resistance is an overall reduction in the production of conidia on plants infected with M. grisea, thereby slowing the epidemic rate of blast.

First Report of a Furovirus Infecting Field-Grown Rye in North America.

Viral symptoms were present in a dwarf recurrent population (99RP17) of rye (Secale cereale) at the North Florida Research and Education Center in Quincy, Gadsden County, FL, during the winter and spring of 2000. Symptoms and distribution of the infected plants in the field were similar to those caused by Soilborne wheat mosaic virus (SBWMV; acronym WSBMV), which was first recognized in North America in 1919 (4) and found in Florida in wheat in 1970 (3). SBWMV has been observed based on symptoms in rye in North America (4). Interveinal, non-continuous, chlorotic areas of leaves and stunting of plants in patchy patterns occurred in four locations (0.8 to 1.6 km between locations). Incidences of the disease ranged from 3 to 15%. Leaves and roots of more than 25 plants were assessed. Using light microscopy, after staining with Calcomine Orange 2RS/Luxol Brilliant Green BL (1), amorphous, vacuolate inclusions were observed in all assayed leaves. With electron microscopy, rigid rods were present with a bimodal distribution of particle lengths that conformed to size distributions found originally in wheat in 1970 in Florida. Leaves with symptoms were sent to Agdia Inc. (Elkhart, IN) and samples were strongly positive for SBWMV using enzyme-linked immunosorbent assay. Cystosori of Polymyxa graminis were detected from a few roots from symptomatic plants. While these associations are suggestive of SBWMV, and rye is a reported host of SBWMV, the possibility of this virus being soilborne rye mosaic virus exists (2). Such a differentiation will require nucleotide sequence analysis. To our knowledge, this is the first report of a furovirus infecting field-grown rye in Florida and in North America. References: (1) R. G. Christie and J. R. Edwardson. 1994. Light and Electron Microscopy of Plant Virus Inclusions Monogr. 9. University of Florida, Quincy. (2) R. Koenig et al. 1999. Arch. Virol. 144:2125-2140. (3) T. A. Kucharek and J. H. Walker. Plant Dis. Rep. 58:763-765, 1974. (4) H. H. McKinney. J. Agric. Res. 23:771-800, 1923.

First Report of Cabbage leaf curl virus (Family Geminiviridae) in Georgia.

Cabbage and collard greens were inflicted with a previously undescribed virus-like disease during the fall 2000. Symptoms on leaves were yellow spots, vein clearing, mosaic, curling, and puckering. Symptomatic plants were widespread in Brooks, Colquitt, Grady, and Pierce counties in Georgia. Disease incidence ranged from 10 to 20% in the majority of the fields surveyed but some fields had 100% incidence. Fields were heavily infested by Bemisia argentifolii and the symptoms were suggestive of a whitefly-transmitted geminivirus infection. A polymerase chain reaction (PCR)-based diagnostic test for geminivirus was conducted. Total DNA was extracted from symptomatic cabbage and collard green plants collected from commercial fields. The two primers, 5'-GCCCACATYGTCTTYCCNGT-3' and 5'- GGCTTYCTRTACATRGG-3' (2,3), are "universal" for genus Begomovirus of family Geminiviridae. The primer pair could amplify a part of the replicase-associated protein and coat protein and the complete common region of DNA-A. The PCR gave a DNA band of expected size (1.1 kb) from both symptomatic cabbage and collard green samples, whereas no such product was obtained from healthy samples, suggesting that the causal agent could be a geminivirus. To establish the identity of the virus, the 1.1 kb PCR product was cloned into pGEM-T Easy (Promega) and sequenced. GenBank search showed that the geminivirus isolated in Georgia was most closely related (98% sequence identity) to Cabbage leaf curl virus (accession number U65529) reported from Florida (1). The virus was mechanically transmitted to healthy cabbage and collard green plants under experimental conditions. To our knowledge, this is the first report of Cabbage leaf curl virus from Georgia. References: (1) A. M. Abouzid et al. Phytopathology 82:1070, 1992. (2) S. S. Pappu et al. Plant Dis. 84:370, 2000. (3) M. R. Rojas et al. Plant Dis. 77:340-347, 1993.

Effect of Silicon Rate and Host Resistance on Blast, Scald, and Yield of Upland Rice.

Blast-resistant, partially resistant, and susceptible cultivars of rice were planted in soil amended with Si at 0, 500, or 1,000 kg/ha at two locations in eastern Colombia to assess differential responses to leaf blast, neck blast, and leaf scald, and to examine the quantity and quality of grains harvested. Leaf and neck blast on partially resistant and susceptible cultivars were reduced by Si as the rate of Si was increased. Depending on the location, the level of severity of leaf and neck blast on partially resistant cultivars, when fertilized with Si at 500 or 1,000 kg/ha, was lowered to that of resistant cultivars without Si. At both locations, yields were increased by as much as 42%, depending on the cultivar, by Si applied at 1,000 kg/ha. In general, high rates of Si reduced the number of broken grains harvested. Grain discoloration, regardless of cultivar or location, was reduced by as much as 70% at the high rate of Si. The application of Si to complement host resistance to blast and scald appears to be an effective strategy for disease management in rice and provides the added benefit of improving the quantity and quality of rice yields.

Partial Characterization of a Distinct Potyvirus Isolated from Watermelon in Florida.

Conspicuous, unusual nuclear inclusions in stained epidermal strips of leaves implicated a virus (designated isolate 2932) as the cause of foliar mosaic in a watermelon plant (Citrullus lanatus) received for analysis from South Florida in 1990. In greenhouse tests, mechanically inoculated plants of Cucurbita pepo (Small Sugar pumpkin and Early Prolific Straightneck squash) and watermelon (Crimson Sweet) developed mosaic or mottle symptoms. Isolate 2932 caused foliar symptoms in 16 cultivars of Cucurbita pepo, including Freedom II and Prelude II, and in six cultivars of watermelon. None of five cultivars of melon (Cucumis melo) or 11 cultivars of cucumber (Cucumis sativus) developed consistent, distinctive symptoms, but all of these cultivars were systemically infected based on back-inoculations to squash. No systemic infection of mechanically inoculated plants of 25 species representing 13 noncucurbitaceous plant families was detected. Crystalline nuclear inclusions, cytoplasmic amorphous inclusions, and cytoplasmic cylindrical inclusions were detected by light and electron microscopy in leaf tissues of infected squash and watermelon. Electron microscopy of squash leaf extracts revealed filamentous particles, and 86% of 159 particles measured ranged from 800 to 890 nm in length. The virus was transmitted in a nonpersistent manner by Myzus persicae from squash to squash in two of three trials. Immunodiffusion tests with polyclonal antisera prepared to partially purified 2932 or its capsid protein showed that the isolate was antigenically different from papaya ringspot virus type W, watermelon mosaic virus 2, and zucchini yellow mosaic virus. In limited testing of field samples of squash and watermelon since 1990, no additional isolates of the 2932 type have been found. The characteristics of isolate 2932 obtained thus far indicate that it is a distinct potyvirus. It is tentatively named watermelon leaf mottle virus to distinguish it from other potyviruses commonly isolated from cucurbits in Florida.

The Association of Severe Epidemics of Cucumber Mosaic in Commercial Fields of Pepper and Tobacco in North Florida with Inoculum in Commelina benghalensis and C. communis.

Since 1995, severe epidemics of cucumber mosaic virus (CMV) have occurred in select fields of tobacco (Nicotiana tabacum) and pepper (Capsicum annuum) in three counties in northern Florida. Yield losses greater than 50% have occurred in both crops. Baker and Zettler (1) identified the presence of CMV in one plant of tropical spiderwort (Commelina benghalensis) in an organic garden on the campus of the University of Florida 10 years ago. In addition, they infected tropical spiderwort and Asiatic dayflower (Commelina communis) with isolates of CMV. Since 1995, in one area of northern Alachua County, Asiatic dayflower has been found in abundance in and around some fields and found to be infected with CMV. Prior to this time, CMV had not been known to be epidemic in any crop in northern Florida. Also, commelinaceous weeds did not occur in such abundance in northern Florida. In Hamilton County, an epidemic of CMV occurred in one field of tobacco in 1997. Tropical spiderwort with viral-like symptoms was growing abundantly in that field. The symptoms in this weed included chlorotic ringspots and chevron-like line patterns. Light microscopy, with Azure A stain, revealed the presence of typical inclusions of CMV in pepper, tobacco, tropical spiderwort, and Asiatic dayflower. Symptomatic samples of the tobacco and the tropical spiderwort reacted in an immunodiffusion test with antiserum to a winged bean isolate of CMV (2). Extracts from tropical spiderwort (isolate 3603) were rubbed on squash. This isolate was thereafter maintained in squash (Cucurbita pepo cvs. Prelude II or Early Prolific Straightneck). Infected plants of both of these cultivars developed strong mosaic symptoms and were stunted. After passage through squash, the 3603 isolate induced mosaic in tobacco (cv. Burley 21). Some plants of the squash cultivars Destiny III and Liberator III, which have transgenic, coat protein-mediated resistance to CMV, developed restricted symptoms after inoculation with this isolate. CMV was recovered by back inoculation from symptomatic plants of these cultivars. Symptomless plants of tropical spiderwort transplanted from the field developed chlorotic ringspots and chevron-like line patterns following inoculation in the greenhouse with isolate 3603. Back inoculations to squash followed by immunodiffusion assays confirmed the presence of CMV in the inoculated tropical spiderwort plants but CMV was not detected in noninoculated control plants. This is the first report of tropical spiderwort being infected with CMV in a commercial situation in the United States. Because commelinaceous plants are well known to be excellent hosts of CMV (1), we believe that the increased presence of perennial, commelinaceous weeds is a factor contributing to the epidemics of CMV in northern Florida. References: (1) C. A. Baker and F. W. Zettler. Plant Dis. 72:513, 1988. (2) C. A. Ku-wite and D. E. Purcifull. Plant Dis. 66:1071, 1982.