Late Blight is caused by the pathogen Phytophthora infestans which originated in Toluca, Mexico.

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​Basic Description:

• Late Blight is caused by and oomycete pathogen known as Phytophthora infestans.

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Host:

• ​ Phytophthora infestans can infect potatoes and tomatoes.

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Distribution:

• ​​​Phytophthora infestans spores are transmitted by seed pieces, wind, rain, and irrigation.

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Symptoms:

• ​Phytophthora infestans infects leaves, stems, tubers and fruits. ​

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Cycle:

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• ​Phytophthora infestans in the Northeast survives in potato tubers saved from last year for seed and in cull piles or soil that did not freeze during the winter.  The pathogen does not overwinter in tomato and is not seed-borne. In more southern regions, the pathogen produces long-lived sexual spores (Oospores) that survive in infected  crop debris and the soil. Sporangia from these southern areas are spread long distances bywind and wind-driven rain. Clouds protect spores being dispersed in wind from the killing effect of ultraviolet radiation. Phytophthora infestans needs cool, wet weather. (2)
• The late blight pathogen has recently undergone changes in Florida that affect disease occurrence there and in other eastern states. Diseased tomato plants in south Florida have survived cold periods in winter allowing the pathogen to persist. Late blight has also been active into the spring as late as May indicating an increased tolerance for warmer temperatures. This means a potential source of inoculum persists until crops are being produced north of Florida and a ‘green bridge’ exists for the pathogen to progress on until it reaches the northeast. (2)

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Control:

• Cultural controls: ​

            Eliminate initial inoculum. The most likely source of late blight is infected tubers that survived the winter in cull piles or compost piles,  tubers that were saved, or tubers that remained in the soil unfrozen and appear as volunteer potato plants.  The late blight organism requires living tissue to survive; it does not survive in the soil or carried tomato seed. Control potato and tomato volunteer plants as well as solanaceous weeds such as hairy nightshade, jimson weed, golden henbane, and others. The ornamental plants petunia and calibrachoa are also hosts for late blight; grow ornamental bedding plants and vegetable transplants in separate greenhouses. Disk under the field or kill with herbicide after harvest is completed.(3)

• Genetic resistance:​ ​​​

             Select resistant varieties were available. ‘Mountain Magic’, ‘Legend’, and ‘Plum Regal’ have excellent resistance to Late Blight. ‘Red Pearl’ and ‘Matt’s Wild Cherry’ are small fruited tomatoes with good resistance. Some heirloom tomato varieties have good tolerance to late blight. A few cultivars also have resistance to Early Blight (Alternaria solani, A. tomatophila) and should be considered. The development of triple (LB, EB, Septoria leaf spot) resistant varieties is underway at Cornell and other Universities. (3)

 

• Biological controls:​

           ​ Rhizobacteria has been shown to suppress late blight in greenhouse-grown potato plants. Many biological control agents have been tested on greenhouse-grown plants. At a study done at the International Potato Center in Lima, Peru they tested Penicillium aurantiogriseum and Stochybotrys atra (a cellulose decaying fungus) applied to the leaves of the potato plants 12 hours prior to inoculating them with P. infestans. P. aurantiogriseum reduced the late blight by 93% and the S. atra reduced late blight by 84%. When the biocontrol agents were applied at the same time as P. infestans it yielded a result of 86% and 65% reduction. While no detailed studies were conducted to figure out why these agents worked a possible explanation is that they had to compete for space and nutrients. Because S. atra is a cellulose decaying fungus it may have compromised the cell wall of P. infestans.
              In another experiment done under field conditions in Germany, a compost tea amended with 7 microorganisms was applied to the foliage of the potato. This did not yield results that were significantly different compared to the use of metalaxyl. ​​​Many P. infestans isolates are resistant to metalaxyl (10).

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Fun Facts:

• France, Canada, USA, and the Soviet Union researched Phytophthora infestans as a biological weapon in the 1940s and 50s. Potato blight was one of more than 17 agents that the United States researched as potential biological weapons before the nation suspended its biological weapons program. (2)
• Late Blight was responsible for the European potato famine in the 19th century, which caused the starvation deaths of more than one million people in Ireland alone. Despite its historic significance, none of the currently grown potato cultivars in the United States have adequate late blight resistance. Currently, late blight is responsible for multibillion-dollar losses annually in both potato and tomato production. (2)
• Forecast Model: Map:http://uspest.org/risk/tom_pot_map

              weather forecasting systems: BLIGHTCAST

Late Blight - Phytophthora infestans