Genotyping of Phytophthora infestans in Eastern Africa Reveals a Dominating Invasive European Lineage.

Strains of Phytophthora infestans, the pathogen causing late blight of potato and tomato, are thought to be moved around the world through infected planting material. Since its first appearance in 1941, late blight has caused important losses to potato production in the eastern-Africa region (EAR). In the current study, the genetic structure of the population in Kenya, Uganda, Tanzania, Burundi, and Rwanda was characterized using 12-plex microsatellite markers with the aim of testing the hypothesis that a strain originating from Europe, 2_A1, has recently dominated the population in EAR. Analyses of 1,093 potato and 165 tomato samples collected between 2013 and 2016 revealed the dominance on potato in all countries of the 2_A1 clonal lineage. On tomato, a host-specialized form of the US-1 lineage appears to persist in Rwanda, Uganda, and Tanzania whereas, in Kenya, most samples from tomato (72.5%) were 2_A1. The US-1 lineage in Tanzania had two private alleles at the Pi02 marker, suggesting a possible independent introduction into the region. US-1 had higher genetic variability than 2_A1, consistent with the earlier establishment of the former. Continuous tracking of P. infestans population changes should help identify new virulent and aggressive strains, which would inform strategic disease management options.

Assessing the Adequacy of the Simulation Model LATEBLIGHT Under Nicaraguan Conditions.

In this study, the adequacy of the late blight simulation model LATEBLIGHT (version LB2004) was evaluated under Nicaraguan conditions. During 2007 to 2008, five field experiments were conducted in three potato-production regions in northern Nicaragua. Two susceptible ('Cal White' and 'Granola') and one resistant ('Jacqueline Lee') potato cultivars were evaluated without use of fungicides and with three application intervals (4, 7, and 14 days) of the fungicide chlorothalonil. The simulation model was considered adequate because it accurately predicted high disease severity in susceptible cultivars without fungicide protection, and demonstrated a decrease in the disease progress curves with additional fungicide applications, similar to that observed in the plots. The model also generally predicted inadequate fungicide control, even with a 4-day spray interval, which also occurred in the field. Lack of adequate fungicide protection would indicate the need for cultivars with higher levels of durable resistance, and that farmers should consider more effective fungicides applications (higher dosages or different chemistries) if susceptible cultivars are used.

Estimating the level of susceptibility to Phytophthora infestans in potato genotypes.

Resistance and susceptibility are closely related terms but differ in their underlying assumptions and measurement. Standardized methods for determining the level of resistance and susceptibility in potato to Phytophthora infestans, the causal agent of late blight, have traditionally been semiquantitative and are not based on a true interval scale, thus making their use in most mathematical and statistical operations inappropriate. Recently, researchers have attempted to develop interval scales using regression analysis of the direct or transformed area under the disease progress curve (AUDPC). In this article, a similar approach is described based on the relative AUDPC (RAUDPC) of one or two reference cultivars and tested using a data set of field trials involving cultivars with varying levels of susceptibility evaluated in different environments in several countries. The coefficient of variation (CV) among trials of the AUDPC was reduced when the RAUDPC was used and even more so when the RAUDPC was made relative to the RAUDPC of cv. Bintje (RaRAUDPC), which was present in all trials. The RaRAUDPC was used in regression models to estimate scale values for eight potato cultivars in 13 to 15 locations (depending on cultivar). The CVs of scale values measuring variation among sites were similar to those of the RaRAUDPC. Using two cultivars gave a slight improvement in CV, which was statistically significant. The scale developed here has ascending numbers for increasing susceptibility, is simple, and can be constructed as a ratio measure, which permitted the calculation of mean, variance, and CV.