Investigating the Host Range of the US-22, US-23, and US-24 Clonal Lineages of Phytophthora infestans on Solanaceous Cultivated Plants and Weeds

Contact Reflectance Spectroscopy for Rapid, Accurate, and Nondestructive Phytophthora infestans Clonal Lineage Discrimination

Populations of Phytophthora infestans, the oomycete causal agent of potato late blight in the United States, are predominantly asexual, and isolates are characterized by clonal lineage or asexual descendants of a single genotype. Current tools for clonal lineage identification are time consuming and require laboratory equipment. We previously found that foliar spectroscopy can be used for high-accuracy pre- and postsymptomatic detection of P. infestans infections caused by clonal lineages US-08 and US-23. In this work, we found subtle but distinct differences in spectral responses of potato foliage infected by these clonal lineages in both growth-chamber time-course experiments (12- to 24-h intervals over 5 days) and naturally infected samples from commercial production fields. In both settings, we measured continuous visible to shortwave infrared reflectance (400 to 2,500 nm) on leaves using a portable spectrometer with contact probe. We consistently discriminated between infections caused by the two clonal lineages across all stages of disease progression using partial least squares (PLS) discriminant analysis, with total accuracies ranging from 88 to 98%. Three-class random forest differentiation between control, US-08, and US-23 yielded total discrimination accuracy ranging from 68 to 76%. Differences were greatest during presymptomatic infection stages and progressed toward uniformity as symptoms advanced. Using PLS-regression trait models, we found that total phenolics, sugar, and leaf mass per area were different between lineages. Shortwave infrared wavelengths (>1,100 nm) were important for clonal lineage differentiation. This work provides a foundation for future use of hyperspectral sensing as a nondestructive tool for pathovar differentiation.

Pathotype Diversity of Phytophthora sojae in Eleven States in the United States

Pathotype diversity of Phytophthora sojae was assessed in 11 states in the United States during 2012 and 2013. Isolates of P. sojae were recovered from 202 fields, either from soil samples using a soybean seedling bioassay or by isolation from symptomatic plants. Each isolate was inoculated directly onto 12 soybean differentials; no Rps gene or Rps 1a, 1b, 1c, 1k, 3a, 3b, 3c, 4, 6, 7, or 8. There were 213 unique virulence pathotypes identified among the 873 isolates collected. None of the Rps genes were effective against all the isolates collected but Rps6 and Rps8 were effective against the majority of isolates collected in the northern regions of the sampled area. Virulence toward Rps1a, 1b, 1c, and 1k ranged from 36 to 100% of isolates collected in each state, while virulence to Rps6 and Rps8 was less than 36 and 10%, respectively. Depending on the state, the effectiveness of Rps3a ranged from totally effective to susceptible to more than 40% of the isolates. Pathotype complexity has increased in populations of P. sojae in the United States, emphasizing the increasing importance of stacked Rps genes in combination with high partial resistance as a means of limiting losses to P. sojae.

Survival of Isolates of the US-22, US-23, and US-24 Clonal Lineages of Phytophthora infestans by Asexual Means in Tomato Seed at Cold Temperatures

Survival of Phytophthora infestans, causal agent of potato and tomato late blight, is thought to be negligible when exposed to freezing conditions typical of a Wisconsin winter. However, the persistence of relatively new P. infestans clonal lineages US-22, US-23, and US-24 within a production region during 2010 to 2014 warranted further investigation. We used tomato seed as a culture medium to determine the survival of P. infestans isolates representing the three lineages under temperatures of 18, 4, 0, -3, and -5°C for 11 time points (1 to 112 days postincubation). Survival varied interactively with temperature, duration of time at a temperature, and clonal lineage of the P. infestans isolate. US-22, -23, and -24 isolates survived for 112 days at 18 and 4°C, 84 days at 0°C, and 14 days at -3°C. US-23 survived longer at -3 and -5°C than did US-22 or US-24. The vigor of US-22 and US-24 isolates decreased with increasing exposure to cold temperatures, a trend that was not observed for the US-23 isolate. By calculating the length of time needed to kill the lineage isolates on infested tomato seed at five temperatures, we predicted that P. infestans would survive in 5% of tomato seed for 99, 25, and 16 days at 0, -3, and -5°C, respectively. We further applied a degree-day model to our empirical data to describe P. infestans survival as a function of cooling degree-day accumulations using archived soil temperatures at 5- and 10-cm depths at four Wisconsin locations over 27 years. The model indicated that survival of P. infestans in 5% of infested tomato seed would occur at 35 and 39% of the location-year combinations at 5- and 10-cm soil depths, respectively. Together, these data suggested that P. infestans has the potential to survive over the winter season by asexual means in infested tomato seed in Wisconsin and other Northern latitudes. Our cooling degree-day model for late blight in the tomato production system offers a tool for anticipating and mitigating disease based on integrated pest management concepts previously utilized for insects.