Prevalence of Myclobutanil Resistance and Difenoconazole Insensitivity in Populations of Venturia inaequalis.

Demethylation inhibitors (DMIs) are a class of single-site fungicides with high levels of protective and curative efficacy against Venturia inaequalis, the causal agent of apple scab. To determine the prevalence of resistance to the DMI fungicide myclobutanil, 3,987 single-lesion conidial V. inaequalis isolates from 141 commercial, research, and baseline orchard populations were examined throughout New England, the mid-Atlantic, and the Midwest from 2004 to 2013. Of these orchard populations, 63% had practical resistance, 13% had reduced sensitivity, and 24% were sensitive to myclobutanil. A sensitivity baseline for the recently introduced DMI fungicide difenoconazole was established to make comparisons with myclobutanil sensitivity in orchard populations. The mean effective concentration of difenoconazole at which mycelial growth was inhibited by 50% (EC50) was determined to be 0.002 µg ml-1 for 44 baseline isolates of V. inaequalis. From 2010 to 2013, 1,012 isolates of V. inaequalis from 37 of the 141 orchard populations above were screened for sensitivity to difenoconazole. In all, 1 orchard population had reduced sensitivity to difenoconazole, while the remaining 36 orchard populations were sensitive to the fungicide. In field experiments, difenoconazole demonstrated high levels of apple scab control on mature apple fruit, despite the fact that the population of V. inaequalis had practical resistance to difenoconazole. Although our results indicate widespread resistance to myclobutanil but not difenoconazole, due to the propensity for cross-sensitivity among DMI fungicides, growers with myclobutanil resistance should be cautious when using difenoconazole for disease management.

Prevalence and Stability of Qualitative QoI Resistance in Populations of Venturia inaequalis in the Northeastern United States.

Quinone-outside-inhibitor (QoI) fungicides are a safe and effective means of managing apple scab caused by Venturia inaequalis. To determine the prevalence of both quantitative (partial) and qualitative (complete) QoI resistance in V. inaequalis in the northeastern United States, we sampled single-lesion conidial isolates (n = 4,481) from 120 commercial and research orchards from 2004 to 2011 with a range of exposure to QoI fungicides from none to several applications a year. In all, 67% of these orchard populations of V. inaequalis were sensitive to QoI fungicides, 28% exhibited QoI practical resistance, and 5% were not sensitive QoI fungicides but had not become practically resistant. Isolates with qualitative QoI resistance, conferred by the G143A cytochrome b gene mutation, were found in 13 of the 34 QoI-resistant orchard populations. To evaluate the stability of the G143A mutation, 27 isolates were selected from different orchard populations to represent the scope of regional populations. These isolates were subcultured continuously in the presence or absence of the QoI fungicide trifloxystrobin. All isolates that initially possessed qualitative resistance maintained the resistant genotype (G143A) for six transfers over 6 months in both the absence and presence of trifloxystrobin. Given the observed QoI resistance in orchard populations of V. inaequalis and the stability of the G143A mutation in individual isolates, apple scab management paradigms must encompass strategies to limit selection of QoI resistance in the sensitive orchard populations remaining in the region.

Etiology of Moldy Core, Core Browning, and Core Rot of Fuji Apple in China.

'Fuji' apple fruit were collected in Shaanxi Province, China, and 186 fungal isolates were obtained from the fruit core region. Fungi were isolated from fruit with symptomless core regions, as well as from the core regions of fruit showing browning, typical moldy core, or core rot. Based on phylogenetic and morphological analysis, all fungi were identified to species. Pathogenicity was determined by cutting apple fruit into halves and daubing spore suspensions containing 1 × 104 up to 1 × 108 spores/ml on the carpel in the core region. Pathogenicity varied significantly among genera, with Alternaria and Cladosporium spp. causing core browning at lower spore concentrations and moldy core at higher spore concentrations. Combinations of pathogens initiated more browning and moldy core than the pathogens applied alone. Epicoccum and Phoma spp. predominated in dry core rot, whereas primarily Trichothecium but also Fusarium and Penicillium spp. caused wet core rot. Core browning was introduced as a new type of core symptom, along with moldy core, dry core rot, and wet core rot. Alternaria alternata, A. tenuissima, A. arborescens, Cladosporium cladosporioides, and C. tenuissimum were the main pathogens causing core browning and moldy core.

Pathogenic diversity of Phytophthora sojae and breeding strategies to develop Phytophthora-resistant soybeans.

Phytophthora stem and root rot, caused by Phytophthora sojae, is one of the most destructive diseases of soybean [Glycine max (L.) Merr.], and the incidence of this disease has been increasing in several soybean-producing areas around the world. This presents serious limitations for soybean production, with yield losses from 4 to 100%. The most effective method to reduce damage would be to grow Phytophthora-resistant soybean cultivars, and two types of host resistance have been described. Race-specific resistance conditioned by single dominant Rps ("resistance to Phytophthora sojae") genes and quantitatively inherited partial resistance conferred by multiple genes could both provide protection from the pathogen. Molecular markers linked to Rps genes or quantitative trait loci (QTLs) underlying partial resistance have been identified on several molecular linkage groups corresponding to chromosomes. These markers can be used to screen for Phytophthora-resistant plants rapidly and efficiently, and to combine multiple resistance genes in the same background. This paper reviews what is currently known about pathogenic races of P. sojae in the USA and Japan, selection of sources of Rps genes or minor genes providing partial resistance, and the current state and future scope of breeding Phytophthora-resistant soybean cultivars.

A New View of Sooty Blotch and Flyspeck.

Sooty blotch and flyspeck (SBFS) fungi colonize the surface wax layer of the fruit of apple, pear, persimmon, banana, orange, papaya, and several other cultivated tree and vine crops. In addition to colonizing cultivated fruit crops, SBFS fungi also grow on the surfaces of stems, twigs, leaves, and fruit of a wide range of wild plants. The disease occurs worldwide in regions with moist growing seasons. SBFS is regarded as a serious disease by fruit growers and plant pathologists because it can cause substantial economic damage. The smudges and stipples of SBFS often result in downgrading of fruit from premium fresh-market grade to processing use. This review describes the major shifts that have occurred during the past decade in understanding the genetic diversity of the SBFS complex, clarifying its biogeography and environmental biology, and developing improved management strategies.

Analysis of Fire Blight Shoot Infection Epidemics on Apple.

Fire blight incidence and spread of the shoot blight phase of the disease was studied in four apple cultivars in replicated blocks over 4 years (1994 to 1997). Cv. York was highly susceptible, followed by 'Fuji' and 'Golden Delicious,' which were moderately susceptible, and 'Liberty,' which was least susceptible. On York, the first appearance of shoot blight was within 48 h of its predicted appearance according to the Maryblyt model in 3 of the 4 years studied. Shoot blight epidemics in York in 1995 and 1996, and Fuji in 1995, were best described with a logistic model that showed apparent infection rates ranging from 0.05 to 0.20, indicating a low to moderately high rate of disease increase. The spatial positions (row and column) of all infected plants in each subplot were recorded on plot maps on each sampling date. The binomial and ß-binomial distributions were fit to the data to test for spatial aggregation of disease incidence for each cultivar plot. Maximum likelihood estimation was possible for 92 (43.6%) of the 211 data sets subjected to this analysis. Of these, 35 data sets were better fit by the ß-binomial distribution than the binomial distribution. The binary power law was used to characterize the relationship between the variance among quadrats within each plot to the variance expected for that plot given the observed level of disease incidence. The binary power law provided an excellent fit to the full data set and to nearly all of the subsets and, with b > 1, indicated that heterogeneity changed systematically with disease incidence. A covariance analysis was conducted to determine the effect of the factors 'year,' 'cultivar,' 'orchard plot,' and 'observation date' on the intercept and slope parameters of the binary power law. In general, plot followed by year had the greatest impact on parameter estimates and is an indication that location and seasonal factors impact heterogeneity of disease, although the specifics could not be ascertained from this study. Ordinary runs analysis was used to analyze the pattern of diseased trees within rows and detected significant nonrandom patterns of disease incidence in 63.5% of the orchard plots over the 4-year study. From these data sets, 68.7% had significantly fewer runs, particularly at disease incidences greater than 0.1. The fewer-than-expected runs at incidences greater than 0.10 provides strong evidence of localized spread.

Production and Dispersal of Conidia of Leucostoma cinctum in Peach and Cherry Orchards Under Irrigation in Eastern Washington.

The objectives of this study were to determine the seasonal production patterns of Leucostoma cinctum conidia in peach and cherry orchards and the effects of irrigation on dispersal of conidia. Conidia were trapped throughout the year and were present in highest numbers during spring and summer on both cherry and peach. The pathogen was dispersed 1.0 m by water drops that simulated irrigation drops that would impact cankers during over-the-canopy irrigation. Spore production and dispersal also resulted from the use of over-the-canopy and under-tree impact sprinklers. Although the two irrigation methods were not directly compared, the latter mode dispersed propagules of L. cinctum over horizontal distances of about 6 m, a distance sufficient to account for the movement of the pathogen between trees within and across orchard rows. It is concluded that disease management could be improved by alteration of horticultural and irrigation practices.

Effect of Inoculum Concentration and Calcium Salts on Infection of Apple Fruit by Botryosphaeria dothidea.

The objective of this study was to determine the effects of calcium salts on growth of Botryosphaeria dothidea and incidence of white rot. The relative virulence of five B. dothidea isolates was determined using the apple cultivars Fuji, Gala, Golden Delicious, Liberty, and York. Cultivar and isolate differences in lesion diameter were significant. Cultivar differences occurred between Fuji, which was most susceptible, and Gala, which was least susceptible. Isolates PA-1 and PA-2 were most virulent. Isolate PA-4 was used at conidia concentrations of 1 × 104, 1 × 105, 1 × 106, and 1 × 107 conidia per ml on Golden Delicious fruit to test the effects of four inoculum concentrations on fruit infection by B. dothidea. Incidence of infection on detached, nonwounded fruit increased as inoculum concentration increased. The effects of four calcium salts on infection of wounded fruit by conidia were examined in field and laboratory experiments. In the field experiments, lesion size was slightly reduced on fruit treated with either calcium hydroxide or calcium silicate after wounding and prior to inoculation with conidia of B. dothidea, whereas lesions that developed at wounds treated with calcium chloride or calcium propionate were similar to those of the control. In the laboratory experiments, in several instances supplementation with calcium salts resulted in increased lesion diameter relative to the control. Two of five isolates tested showed increased percent germination with all four calcium salts, and one isolate showed increased germination in the presence of two of the four calcium salts. Germ tube elongation was not affected by calcium salts for four out of five isolates tested. There was significant variation among isolates and calcium salts on the growth of B. dothidea in liquid culture. Calcium chloride and calcium hydroxide enhanced the growth of three isolates and had no effect on the other three isolates. Calcium propionate inhibited the growth of four isolates and had no effect on the other two isolates. Calcium silicate inhibited the growth of one isolate and had no effect on the other five isolates.

Effects of Calcium Salts on Apple Bitter Rot Caused by Two Colletotrichum spp.

The effects of three calcium salts on conidial germination, germ tube elongation, growth in vitro, and infectivity in the laboratory and field were studied for the apple bitter rot pathogens, Colletotrichum gloeosporioides and C. acutatum. Calcium chloride, calcium propionate, and calcium silicate at 1,000 µg of calcium per ml had no effect on conidial germination; however, calcium chloride and calcium propionate inhibited germ tube growth by 41 and 50% relative to the control, respectively. All three calcium salts reduced fungal dry weight in liquid culture media. When calcium salt solutions were applied to wounded apples prior to inoculation, fruit treated with calcium chloride and calcium propionate exhibited 30% smaller lesions than those treated with calcium silicate or the control, which were similar. Fruit treated with calcium chloride and calcium propionate exhibited delayed formation of acervuli relative to the control and calcium silicate, which were similar. When fruit were inoculated with varying concentrations of conidia, from 1 × 103 to 1 × 105 conidia per ml, fruit treated with calcium chloride exhibited reduced incidence of infection after inoculations with 1 × 103 conidia per ml. In all tests at 1 ×104 and 1 × 105 conidia per ml, the control and calcium salt treatments resulted in similar incidences of infection after inoculations. In six field trials, where fruit received three weekly dilute applications of calcium solutions, fruit treated with calcium salts and then inoculated with conidia of either C. gloeosporioides or C. acutatum exhibited lower incidences of infection when compared with control fruit. These experiments demonstrate that calcium salts have suppressive activity against the bitter rot pathogens and could be used as part of a disease management program.

Assessment of an Alternative IPM Program for the Production of Apples for Processing.

Conventional and alternative integrated pest management (IPM) programs for managing arthropods and diseases affecting processing apple production were compared over 4 years. The effects of the two programs on populations of pest and beneficial insects, and on disease incidence and severity, were evaluated in the field and laboratory and at a commercial fruit processing plant by federal inspectors. The economic implications of the alternative management program were assessed by examining its relative costs and benefits compared with conventional management. In the alternative program, arthropods were managed with oil and Bacillus thuringiensis (Bt), and early-season diseases were managed with fungicides. Calcium chloride was used to suppress rot diseases. The alternative plots showed lower levels of indirect pests (aphids, leaf-hoppers, and mites) and increased numbers of arthropod natural enemies. On harvested fruit, there was significantly more injury in the alternative plots from codling moth or oriental fruit moth, plum curculio, and apple maggot. Levels of foliar diseases were similar between treatments; however, fruit from the alternative program exhibited significantly more sooty blotch, flyspeck, and rots. At the commercial processor, levels of cull fruit were higher for the alternative program in 2 of the 3 years with harvestable yield. Loads of fruit that were not acceptable for processing were due mostly to internal worms and worm damage. Incidence of decay was within the quality limits set by the processor, except for one conventional sample, in which decay exceeded 5%. Of the 36 loads of fruit that were examined from each program, 4 and 13 loads were rejected from the conventional and alternative programs, respectively. Net present value comparisons were dependent upon how rejected loads were valued. At cider prices for rejected loads, net returns were similar between programs. When rejected loads were valued at a complete loss, conventional net present value was over 140% higher than the alternative program. Among cultivars, Golden Delicious was most profitable and York was least profitable.