Hilary Mayton

Quantifying the Rate of Release and Escape of Phytophthora infestans Sporangia from a Potato Canopy

ABSTRACT A means for determining the rate of release, Q (spores per square meter per second), of spores from a source of inoculum is paramount for quantifying their further dispersal and the potential spread of disease. Values of Q were obtained for Phytophthora infestans sporangia released from an area source of diseased plants in a potato canopy by comparing the concentrations of airborne sporangia measured at several heights above the source, with the concentrations predicted by a Lagrangian Stochastic simulation model. An independent estimate of Q was obtained by quantifying the number of sporangia per unit area of source at the beginning of each sampling day by harvesting diseased plant tissue and enumerating sporangia from these samples. This standing spore crop was the potential number of sporangia released per area of source during the day. The standing spore crop was apportioned into time segments corresponding to sporangia concentration measurement periods using the time trace of sporangia sampled above the source by a Burkard continuous suction spore sampler. This apportionment of the standing spore crop yielded potential release rates that were compared with modeled release rates. The two independent estimates of Q were highly correlated (P = 0.003), indicating that the model has utility for predicting release rates for P. infestans sporangia and the spread of disease between fields.

Oospore Survival and Pathogenicity of Single Oospore Recombinant Progeny from a Cross Involving US-17 and US-8 Genotypes of Phytophthora infestans

Oospores of Phytophthora infestans produced in vitro and in planta, from a cross between US-17 and US-8 genotypes, were exposed to a variety of environments and their survival was assessed. Additionally, the pathogenic characteristics of some resultant progeny isolates were assessed. Viability of oospores as measured by plasmolysis declined slightly over a period of 18 months whether they were stored in water at 4°C, in soil at 18°C, or in soil under natural field conditions. In comparison, viability as measured by germination was lower overall but appeared to increase after storage in soil. Oospores produced in planta were buried in the field in the fall of 1998, and were capable of infecting both tomato and potato leaflets when recovered in May 1999. Single oospore progeny (n = 53) from the in vitro cross were analyzed individually for genetic and pathogenicity characteristics. All 53 progeny tested for restriction fragment length polymorphisms with probe RG57 were hybrids. All but one progeny produced sporulating lesions on detached potato or tomato leaflets in growth chamber tests, but most lesions were smaller and developed more slowly than those produced by either parental isolate. In a further test of pathogenicity, under field conditions, none of a subset of 10 A2 progeny was capable of initiating a detectable epidemic in small plots of either potatoes or tomatoes.

Resistance to Phytophthora infestans in Lycopersicon pennellii

To determine if the desert tomato, Lycopersicon pennellii, possesses resistance to late blight, caused by Phytophthora infestans, two plant populations were analyzed. Resistance was identified through assessments of disease progress in an F2 mapping population (L. esculentum × L. pennellii) and in a series of introgression lines (L. pennellii into L. esculentum). Levels of resistance varied widely among individuals within each population. However, the response of individuals to different strains of P. infestans was consistent. In the mapping population, a quantitative trait locus (QTL) was detected near marker T1556 on chromosome 6. This QTL accounted for 25% of the phenotypic variance in the population. The occurrence of this QTL was confirmed from analysis of the introgression lines (ILs), where IL 6-2 (containing marker T1556) was the most resistant IL in 2002 and the second most resistant IL in 2001. The identification of an additional QTL for resistance to late blight in tomato will aid in the development of durable resistance to this devastating disease.

Quantitative resistance to late blight from Solanum berthaultii cosegregates with RPi-ber: insights in stability through isolates and environment.

Genetic resistance is a valuable tool in the fight against late blight of potatoes but little is known about the stability and specificity of quantitative resistance including the effect of defeated major resistance genes. In the present study we investigated the effect of different isolates of Phytophthora infestans on the mode of action of RPi-ber, an R-gene originating from Solanum berthaultii. The experiments were conducted on progenies derived from two reciprocal inter-specific backcrosses of Solanum tuberosum and S. berthaultii. The plant–pathogen interaction was tested in diverse environments including field, greenhouse and growth chamber conditions. The RPi-ber gene provided complete resistance against a US8 isolate of P. infestans in all trials. When isolates compatible with RPi-ber were used for inoculation, a smaller, but significant resistance effect was consistently detected in the same map position as the R-gene. This indicates that this R-gene provides a residual resistance effect, and/or that additional resistance loci are located in this genomic region of chromosome X. Additional quantitative resistance loci (QRL) were identified in the analyzed progenies. While some of the QRL (such as those near TG130 on chromosome III) were effective against several isolates of the pathogen, others were isolate specific. With a single exception, the S. berthaultii alleles were associated with a decrease in disease severity. Resistance loci reported in the present study co-locate with previously reported R-genes and QRL to P. infestans and other pathogens.

The Roles of Three Fungicides in the Epidemiology of Potato Late Blight

Three fungicides were tested in the field for efficacy on late blight caused by Phytophthora infestans. The effects of these fungicides on epidemic development, lesion growth rate and sporulation were measured. No fungicide completely arrested epidemic development under the environmental conditions of these experiments. However, the fungicide mixture, propamocarb hydrochloride plus chlorothalonil, had the most suppressive effect of the fungicides tested. The mechanism of effect included suppression of disease progress and lesion expansion. Growth chamber studies demonstrated that 24°C compared to 10 or 16°C limited cymoxanil efficacy.

Environmental and genetic factors influencing self-fertility in Phytophthora infestans.

ABSTRACT Phytophthora infestans is generally regarded as heterothallic-requiring physical proximity of two individuals of different mating type (A1 and A2) for oosporogenesis. Recent reports of limited selfing in young cultures of this oomycete stimulated us to investigate factors contributing to the phenomenon. The ability to produce oospores rapidly (within 2 weeks) in pure, single individual cultures (self-fertility) was tested in 116 individual isolates. The 116 isolates were from geographically diverse locations (16 countries) and were genetically diverse. Mating type and growth medium were the most prominent factors in determining if an isolate would be self-fertile. The majority of A2 isolates (45 of 47 tested) produced oospores when grown on a 50:50 mixture of V8 and rye B medium. In contrast, the majority of A1 isolates (65 of 69 tested) did not produce oospores on this medium. None of the 116 isolates produced oospores when grown on rye B medium (with no V8 juice). Further tests on representative A1 and A2 isolates revealed that oatmeal agar, tomato juice agar, and V8-juice agar all induced the A2 mating type isolate to produce oospores but did not induce the A1 mating type isolate to produce oospores. Calcium carbonate and pH did not alter the self-fertile oospore production in either A1 or A2 mating type isolates. For in vivo tests, the application of fungicide to potato or tomato leaf tissue either before or after inoculation did not stimulate any individual isolate (one A2 and one A1 isolate) to produce oospores in infected tissue. However, in all of the controls for all experiments (in vivo and in vitro), many oospores were produced rapidly if both strains grew in physical proximity.

Impact of Growing Environment on Anthracnose Severity of Switchgrass Cultivars and Clones

Anthracnose (caused by Colletotrichum navitas) has the potential to significantly reduce biomass yield of switchgrass (Panicum virgatum L.); however, limited information is available on the impact of growing environment on tolerance of switchgrass to anthracnose. Therefore, the major objectives of this study were to (i) examine genotype-environment (G × E) effects on anthracnose severity in populations of switchgrass cultivars and individual genotypes and (ii) determine clonal repeatability estimates and stability analysis of anthracnose tolerance on individual switchgrass genotypes. Two experiments were conducted at one prime and two marginal soil locations in New Jersey. In all, 14 switchgrass cultivars were established from seed in 2008 for experiment 1 and 50 replicated switchgrass clones were planted in 2009 for experiment 2 at all three locations. Anthracnose was rated visually in 2010 for experiment 1 and in 2010 and 2011 for experiment 2. Significant G × E interactions were detected for both experiments (P ≤ 0.05) and anthracnose severity varied by location and cultivar. Clonal repeatability estimates for disease tolerance among clones was 0.78 on a clonal basis and 0.32 on a single-plant basis. Lowland ecotypes exhibited less disease overall than upland ecotypes. Results from this study indicate that selection for improved tolerance to anthracnose should be conducted after evaluation across several environments over multiple years.