Virulence and Mating Type of Phytophthora infestans Isolates in the Czech Republic

To evaluate the frequency and stability of the occurrence of P. infestans races and mating types in the Czech Republic, 338 monosporic isolates were collected from 31 sites in different potato-growing areas from 2012 to 2014 and in 2016. In total, 142 isolates were evaluated for virulence and race structure using the detached leaflet assay on Black’s differential set, supplemented with cultivar Sarpo Mira and somatic hybrid REG 46F. With the exception of virulence for resistance genes R9 and Rpi-blb-1, all virulence genes were detected among isolates, with a predominance of genes R1, R3, R7, R10, and R11. Most isolates were virulent to five or more R-genes, with a mean virulence complexity of 7.1. Among the 38 races detected, the most commonly occurring races were 1.2.3.4.6.7.10.11 and 1.2.3.4.7.10.11. Of the 338 isolates tested by the pairing test and the cleaved amplified polymorphism sequence (CAPS) marker, 40% were of the A1 mating type and 60% were of the A2 mating type, with an A1 : A2 isolate ratio demonstrating the predominance of the A2 mating type each year of the survey.

Three Inoculation Methods for Evaluating Sclerotinia Blight Resistance in Peanut

ABSTRACT Laboratory-based assays for screening germplasm for resistance to Sclerotinia blight in peanuts can be conducted year-round, and thus may accelerate progress in breeding for resistant plants. Three previously proposed inoculation methods (using main stems of intact plants, detached main stems, or detached leaflets) were compared on six peanut genotypes known to represent range of resistance to Sclerotinia blight in the field or laboratory. The intact plant and detached main stem assays identified the most resistant and susceptible genotypes, but different results were obtained from either assay with Red River Runner, a cultivar with intermediate resistance to Sclerotinia blight. No differences among genotypes were observed with the detached leaflet assay. The sensitivity ratio was used to compare the three inoculation methods and identified the intact plant assay as the method with the smallest error variance. These results help identify the most efficient method for assaying physiological resistance to Sclerotinia blight in peanut.

Detached-Leaflet Evaluation of Tomato Germplasm for Late Blight Resistance and Its Correspondence to Field and Greenhouse Screenings

Breeding for disease resistance requires efficient techniques for screening large plant populations. Late blight (LB), caused by the oomycete Phytophthora infestans, is one of the most devastating diseases of tomato (Solanum lycopersicum) worldwide, and there is a great interest in developing cultivars with resistance to this pathogen. Screening for LB resistance is commonly conducted under field or greenhouse conditions using whole plants. In a previous study, we demonstrated correspondence between field and greenhouse screening of tomato for LB resistance. Here, we report the use of a detached-leaflet assay for such screening. Seventy-two genotypes from two tomato species, varying in degree of resistance and susceptibility to LB, were evaluated in two replicated experiments for response to LB in a detached-leaflet assay, and the results were compared with those previously obtained from field and greenhouse screening of the same genotypes. There were significant (P < 0.001) positive correlations between replications (average r = 0.75) and experiments (average r = 0.72), suggesting that the detached-leaflet experiments were consistent. Further, there were significant (P < 0.001) positive correlations between responses in the detached-leaflet assay and those from field (r = 0.82) and greenhouse screenings (r = 0.84), suggesting reliability of the detached-leaflet assay. The results indicate the utility of the detached-leaflet assay for evaluating tomato for LB resistance, which may facilitate screening of large breeding populations.

Sclerotinia Blight in Georgia and Evidence for Resistance to Sclerotinia sclerotiorum in Runner Peanuts

Sclerotinia blight (Sclerotinia sclerotiorum (Lib.) de Bary) was recently identified in a commercial peanut (Arachis hypogaea L.) field in Appling County, GA. Symptoms were first observed on the cultivars Tifrunner and Georgia 02C. Plant inoculations and a detached leaflet assay were conducted to determine the susceptibility of the cultivars Georgia Green, Georgia 02C, Georgia 03L, AP-3, Georgia 01R, Hull, C-99R, and Tifrunner. For plant inoculations, lesion lengths were greatest for Okrun, the susceptible control, and Georgia 02C; lesion lengths for C-99R and Georgia 01R did not differ significantly from Tamspan 90, the resistant control. Georgia Green, the current commercial standard, exhibited intermediate lesion lengths. Similar results were obtained from the detached leaflet assay. These results suggest that differing levels of resistance to S. sclerotiorum are available in runner cultivars used in the southeastern United States. Accepted for publication 27 March 2006. Published 31 May 2006.

Aggressiveness of Isolates of Phytophthora infestans from the Columbia Basin of Washington and Oregon

The aggressiveness of 22 isolates of Phytophthora infestans collected from naturally infected potato plants in the Columbia Basin of Washington and Oregon was determined on detached potato leaflets at 18°C in an incubator. Selected isolates were evaluated on whole plants in a greenhouse. Aggressiveness was measured by using the area under the lesion expansion curve (AULEC), incubation period, latent period, sporulation capacity, and lesion size on detached leaflets and the area under the disease progress curve and sporulation capacity on whole plants. The detached-leaflet assay was useful in that a large number of isolates were tested, several components of aggressiveness were studied, and significant differences were found among isolates. Significant variation for components of aggressiveness was found within and among isolates classified according to genotype. Significant interactions among isolates and cultivars were found for some components of aggressiveness, so results were pooled according to cultivar. On average, US-8 and US-11 isolates had higher AULEC scores, indicating aggressiveness higher than that of US-7, US-6, and US-1 genotypes. One US-8 genotype isolate had a higher standardized sporulation capacity than isolates of the other genotypes. US-6 genotype isolates were the least aggressive group, as indicated by low AULEC, sporulation capacity, and lesion size values. The replacement of the US-1 genotype by the US-8 genotype in the Columbia Basin may be partially explained by the increased aggressiveness of US-8 isolates. Additionally, potato growers may need to shorten intervals between fungicide applications and begin applications earlier.