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United States Department of Agriculture

Agricultural Research Service

Research Project: Identifying and Characterizing Resistance from Diverse Potato Germplasm Sources to Highly Aggressive Strains of Late Blight

Location: Vegetable Crops Research Unit

2013 Annual Report


1a.Objectives (from AD-416):
In 2009, a new late blight strain US-22 was identified along the East coast and as far west as Wisconsin. This strain was virulent on potatoes and significantly impacted potato production in affected areas. There was greater variation in the strains identified in potato fields in the U.S. in 2010, however US-22 was again identified in WI along with US-23 and 24. Although not yet present in the USA, destructive and highly virulent strains of P. Infestans have spread through Europe (genotype Blue 13), and are present in Guatemala (currently uncharacterized). These new strains can be much more aggressive compared to the old populations. We need to be prepared with germplasm that is resistant to highly virulent strains.

Specific objectives:

1. Conduct transformations with candidate sequences of a putative S. microdontum R-gene.

2. Identify functional orthologs of the late blight R-gene RB from disease resistant wild germplasm and test the functionality of these genes using a transient expression assay.

3. Evaluate the late blight resistant potato advanced breeding lines including the varieties 'Defender' and 'Jacqueline Lee' for resistance to Blue 13 and identify the presence of genes that recognize P. infestans effectors.

4. Evaluate the effect of pyramiding the RB with conventionally bred late blight resistant lines.


1b.Approach (from AD-416):
Test the resistance phenotype of transgenic plants with the S. microdontum gene to P. infestans isolates collected from the United States and Guatemala.


3.Progress Report:

This project was renumbered from 3655-21000-049-17S to 3655-21220-002-04S. The objective of this project is to evaluate the ability of oospores formed by new clonal lineages of P. infestans to overwinter under field conditions and survive different freezing temperatures under laboratory conditions. For evaluation of overwintering, an experiment of a completely randomized design with 3 treatment and 3 replicates was used; treatment 1: combination of US-22 x US-23, treatment 2: combination of US-22 x US-23, and treatment 3: no oospores (negative control). Each replicate consisted of one bucket containing 2 pots, each pot containing 500 g of soil and 10,000 oospores. Buckets were placed in an experimental field at Hancock Agricultural Station, from the second week of November to the second week of April during the winter 2011-2012 and 2012-2013.

For the evaluation of oospores survivability, three different concentrations of oospores from each combination of mating types were used. The oospore concentrations were 100, 1,000, and 10,000 in 100 g of soil. Incubation under 5 different temperatures: 22, (room temperature), 0, -10, -20, and -80°C, for periods of time ranging from 1 to 4 months. Infested soils were sampled at each temperature and time period to assess infectivity of oospores. Minitubers were planted into infected soils to assess infectivity of the oospores. This experiment was performed in winter-spring 2011-2012 and infectivity was evaluated. A second experiment is underway in winter-spring 2012-2013. Our results showed that potato plants from minitubers planted in infested soil with oospores from the winter 2011-2012 did not show any symptoms of late blight after 60 days of planting. This research relates to objective 4 of the project through the determination of the overwintering capabilities of P. infestans strains and objective 5 through the determination of P. infestan's ability to form oospore progeny through sexual recombination.


Last Modified: 10/30/2014
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