Strain-specific late blight forecasting

Strain-specific late blight forecasting

Summary

Late blight caused by the pathogen Phytophthora infestans, has been a major threat to global food security ever since the Irish famine of the 1800’s. In the US, late blight is potentially important on nearly all of the approximately 1.2 million acres of potato production. Over 2000 tons of fungicide (active ingredient) are used annually on potato, typically spread over 8-18 applications. The total global cost of the disease is ~$7 billion per year. On tomatoes, the disease can be and has been equally devastating. The most recent example occurred in 2009 where the late blight pandemic in the mid-Atlantic and northeast regions eliminated tomato plants in many organic farms and home gardens I have characterized phenotypes of the four most important strains of this pathogen, and this information has been used by growers throughout the Northeast to make informed management decisions. The goal of this project is to characterize a novel set of twenty genotypically diverse strains of P. infestans that were detected in the Northeastern US in the years 2010 and 2011 (Table 1) in the event that any one of these might become a dominant strain in the future. More specifically, I am interested in determining host preference, sensitivity to mefenoxam, and the effect of temperature on the rate of germination and on mycelial growth. So far I have tested the sensitivity of these isolates to the systemic fungicide, mefenoxam, and have found diversity within this trait (sensitive, intermediate and resistant isolates). Phenotypic analysis may take weeks to months, but if DNA genotyping assigns strains to a lineage of known phenotype, there can be results within a day. The data I am gathering now, will enable rapid appropriate response to any of the twenty recently emerged isolates in the northeastern US. This information will be used in a disease forecast hosted on the Cornell Decision Support System. This DSS is available to anyone in the Northeast at http://blight.eas.cornell.edu/blight_lab/.

• Table 1. Phytophthora infestans isolates investigated in this study.

Objectives/Performance Targets

My overall objective is to provide the data necessary to construct a strain-specific forecast for late blight of potato and tomato. This will be achieved through four specific sub objectives:

1. 1. Determine sensitivity to an important fungicide (mefenoxam) of P. infestans strains collected in the Northeastern US in the years 2010 and 2011. This objective has been completed. Differences in sensitivity to mefenoxam were seen among isolates.

2. 2. Determine host preference for new genotypes of P. infestans strains collected in the Northeastern US in the years 2010 and 2011. I am currently in the process of determining host preference for twenty recently emerged isolates of P. infestans. I have initiated the first replicate of this assay for a subset of the isolates.                 

3. 3. Determine the effect of temperature on the rate of germination for new genotypes of P. infestans strains collected in the Northeastern US in the years 2010 and 2011. I am currently in the process of determining the effect of temperature on the rate of germination for twenty recently emerged isolates of P. infestans. I have initiated the first replicate of this assay for a subset of the isolates.          

4. 4. Determine the effect of temperature on incubation period (time between inoculation and the appearance of typical late blight symptoms), latent period (time between inoculation and the start of the production of spores), sporulation capacity and lesion area. I have not yet initiated this objective. Because the purpose of this objective is to understand the effect of climate change on the development of the pathogen, an additional sub objective will be included, where I am going to test the effect of temperature on mycelia growth. This assay will be done by exposing the different strains of P. infestans (growing on pea agar), to each of four different temperatures (10, 15, 20 and 25 ºC).

Accomplishments/Milestones

Over the past semester I have finished assessing the sensitivity of twenty diverse genotypes of Phytophthora infestans. These P. infestans isolates weredetected near central and western New York State in 2010 and 2011.Mefenoxam sensitivity was assessed as described previously by Therrien et al., (1993){Therrien, 1993 #395;Therrien, 1993 #5}, except that mefenoxam was used in place of metalaxyl. Strains were grown on pea agar amended with Ridomil Gold EC (Syngenta, Greensboro, NC) such that the final concentrations of the active ingredient (mefenoxam) were 0, 5, or 100 ug mL^-1. Mycelial plugs (8 mm diameter) were obtained from actively growing cultures, transferred to the test plates and incubated for approximately 10 to 12 days, or until growth on the control mefenoxam plate (0 ug mL^-1) was approximately 75 to 90% of the diameter of the petri plate. Assessment of mefenoxam sensitivity was determined on the basis of radial growth of cultures grown on plates amended with mefenoxam (5 or 100 µg mL^-1) compared to non-amended controls. Growth on mefenoxam-amended plates, 5 and 100 µg mL^-1, was represented as a proportion of the growth on the non-amended control plates (Figure 1).

The isolates tested, differed in their sensitivity to mefenoxam. Isolates GDT-01, -04, -05, -06, -08, -12, -13, -15, -16, -18, -19 and -20 seem to be sensitive to mefenoxam. Their growth on mefenoxam-amended plates (5 µg ml^-1 and 100 µg ml^-1) was less than 40% relative to the control (0 µg ml^-1). Isolates GDT-02, -03, -07, -09, -10, -11, -14, and -17 on the contrary range from intermediate to resistant. In general they grew more than 40% relative to the control on mefenoxam-amended plates with 5 µg ml^-1 but less than 40% on mefenoxam amended plates with 100 µg ml^-1. Refer to Table 1 for isolate information.

The upcoming semester I will be determining host preference and the effect of temperature on germination rates and mycelial growth. After determining these characteristics more specific experiments will be conducted to determine the effect of temperature on incubation period (time between inoculation and the appearance of typical late blight symptoms), latent period (time between inoculation and the start of the production of spores), sporulation capacity and lesion area.

So far the project has progressed as planned. As expected the variability when working with living organisms is high and therefore enough replicates must be performed to make real sense of the data. I intend to have at least three biological replicates for all of my assays.

Reference:

Therrien, C. D., Tooley, P. W., Spielman, l. J., Fry, W. E., Ritch, D. L., and Shelly, S. E. 1993. Nuclear DNA content, allozyme phenotypes and metalaxyl sensitivity of Phytophthora infestans from Japan. Mycological Research. 97:945-950.

• Figure 1. Response of Phytophthora infestans isolates to mefenoxam. Relative growth (as percentage of control) at 5 µg ml-1 (top) and 100 µg ml-1 (bottom) relative to control 0 µg ml-1. Sample sizes for each multilocus genotype can be found in Table 1. Bars represent one standard error from the mean.

Impacts and Contributions/Outcomes

The data I am gathering now, will enable rapid and appropriate disease management strategies against any of the twenty recently emerged isolates of Phytophthora infestans in the northeastern US. The information that I will gather through this project will be useful in the event that any one of these twenty isolates might become a dominant strain in the future. The assessment for mefenoxam sensitivity indicated that these isolates are phenotypically diverse. Given that phenotypic analysis may take weeks to months, it will be important to learn the characteristics of each of these strains to be able in the future through rapid genotyping techniques to be able to assign strains to a lineage of known phenotype.

The ultimate goal of this project is to incorporate the information gathered for each of the isolates studied in a disease forecast hosted on the Cornell Decision Support System. This DSS is available to anyone in the Northeast at http://blight.eas.cornell.edu/blight_lab/.

Collaborators: