Effectors for Change

A USDA-funded project to improve control of late blight

Published in the April 2012 Issue Published online: Apr 09, 2012 Howard Judelson, professor, Department of Plant Pa
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Late blight lesion on a potato leaflet. Photo courtesy of Amanda GevensA perennial worry for potato growers is late blight, a disease well-known for its potential to devastate crops of potato as well as tomato. To help manage this infamous disease, 18 universities and research institutes have teamed together in an undertaking funded by USDA's National Institute of Food and Agriculture.

The project combines research, extension and education to help ensure the sustainable and long-term control of the disease, while reducing on-farm costs and fungicide use. Goals include improving the efficiency of chemical control as well as developing plant-based solutions. Having just completed the first year of its five-year funding period, the group has already released some tools for growers and will roll out additional resources in the next few years.

Cyber Attack

One of the first developments was a national disease reporting and alert website, located at http://USAblight.org. The site provides information on late-blight outbreaks, phone and email alerts to registered users and tools for forecasting disease. The latter includes a Decision Support System that uses a variety of inputs, including weather, to help growers model strategies for managing disease risk using different fungicide regimes.

The USAblight.org site supplements existing state-specific tools by providing information on a nationwide basis and to regions lacking their own resources. The site also helps to network extension personnel from multiple states. The site tracks both potato and tomato late blight, which is important since both plants can be infected by most strains of the pathogen, Phytophthora infestans. Growers are encouraged to report outbreaks on either host, and instructions on making reports can be found on the USAblight.org site.

Registration with USAblight.org also allows users to submit suspected late-blight lesions for analysis, including DNA-based genotyping of the P. infestans strain. Currently only a few genotypes of P. infestans exist in the U.S., but these may vary between regions and over time. The displacement of the US-1 genotype in the U.S. by new forms such as US-8 in the 1990s was particularly vexing since the new strains were more aggressive and resistant to the fungicide metalaxyl (mefenoxam). US-8 still persists in some regions, but in recent years new genotypes have appeared, termed US-22, US-23 and US-24.

Knowing which genotypes are present can help inform strategies for managing disease risk. For example, a grower would benefit by knowing that US-22 was in their area, since that genotype is not very aggressive on potato and is sensitive to metalaxyl. In 2011, about 100 samples were submitted by growers across the country and genotype data was transmitted rapidly to the providers.

One advantage of using DNA methodology to characterize P. infestans is that trait information can be predicted in just a few days, assuming that a previously characterized genotype is involved. By comparison, conventional analysis of P. infestans from a lesion can take several weeks. The Decision Support System available through USAblight.org is now being modified to incorporate genotype information and will be released to the public.


DNA Assays

A longer-term goal of the project is to develop DNA assays predictive of pathogen traits relevant to disease management, based on the chromosomal loci from P. infestans that determine those characters. These include aggressiveness on potato and tomato, the ability to overcome plant resistance genes, temperature optimum for spore germination and sensitivity to the available chemistries for controlling late blight. This research is enabled by recent advances in resources for studying the genetics of P. infestans , including the initial sequencing of its genome in 2009 and the subsequent development of DNA sequencing technologies that have one-hundredth the cost of prior methods.

Optimizing delivery of information about late blight to growers is also key to the project. Late blight is a community disease, spreading readily between farms. In many regions, the community includes large and small farms, organic and conventional operations and growers with varying levels of experience. Since all need to coordinate for optimal disease control, the project includes social scientists who identify the best methods to disseminate information and advice about late blight. The team also includes an economist that will assess adoption rates and fiscal impacts of the project outputs.

The project also will develop plant-based solutions for battling late blight, through a promising new method for screening germplasm collections for sources of resistance. The products of plant resistance genes interact directly or indirectly with pathogen proteins called "effectors," which include what traditionally have been named avirulence proteins. Based on genome sequencing, it is now known that each isolate of P. infestans contains genes for about 500 such proteins, many of which differ between strains. This variation explains why most resistance genes lack durability. By sequencing genomes of multiple isolates of P. infestans , conserved effectors can be identified, produced in the laboratory and tested on potential sources of resistance. This should speed the delivery of durable resistance to growers.

Manipulating genes in plant defense response pathways is also being explored to improve resistance. It may only be practical to implement such solutions by genetic engineering, which has received resistance in many countries. However, consumer acceptance of biotech solutions is a moving target and consequently the project also includes a survey of consumer attitudes in the U.S.

By taking a multifaceted strategy, this project aims to develop a range of solutions for growers. Not all may transition successfully from the lab to field, but multiple approaches need to be tested since there may not be one "silver bullet" against late blight. The project also engages undergraduates in research and extension related to late blight, to develop the next generation of agricultural scientists that will help potato growers battle disease and maximize profits.