AgronomyDisease Watch: Late Blight

Disease Watch: Late Blight


Late blight disease caused by Phytophthora infestans devastates potato foliage and tubers. The pathogen can survive between growing seasons as mycelium in potato tubers and plant tissues, and on alternative hosts of Solanaceae family. The pathogen may also overwinter as oospores in soil. Infected tubers used for seed or discarded onto cull piles or infected volunteer potatoes are sources of infection for the new growing season. In addition to potatoes, tomatoes, eggplant, peppers, petunias and other solanaceous plants sold at garden centers or planted in home gardens are also potential sources of late blight.

Though late blight most commonly occurs in cool, wet climates, it can also occur anywhere when irrigation or wet conditions combine with cool temperatures to favour disease development. The late blight pathogen does not require stressed plants to initiate the disease. Under favourable weather and crop conditions, a potato field can be defoliated in two to three weeks, and sometimes in a matter of days, due to late blight.

Late blight is the most important biotic constraint to potato production globally. The disease caused an epidemic which broke out in Europe in 1845. Although the attack took Europe completely by surprise, blight had already been ravaging the potato crops in North America in the previous two seasons. Potato late blight caused massive food insecurity during Ireland’s famine of the 1840s which resulted in the death or emigration of over two million people from Ireland. In addition, the disease spurred the emergence of the field of plant pathology. During studies to understand the cause of the disease, botanist Anton deBary demonstrated that the fungal-like growth observed on blighted plants was the cause of late blight. His work led other scientists to investigate other fungi and bacteria associated with plant disease and resulted in the identification of the causal pathogens of many plant diseases.

Prior to the mid-1990s, all Phytophthora infestans strains in the U.S. and Canada were of the A1 mating type (US-1 or US-11), which was less pathogenic than the new strains. During the 1990s, more aggressive and genetically diverse strains of the late blight pathogen migrated to Canada and the United States. For example, US-8 strain became predominant and was found to be more aggressive in potato than previous strains.

Late blight has been referred to as a “community disease,” due to its ability to spread rapidly from field to field under the right weather conditions.  Blight spores travel easily on the wind when the weather is cool and moist, and can rapidly infect neighbouring fields. As such, understanding the symptoms of the disease and what to do when it is detected are essential to preventing an outbreak from rapidly turning into an epidemic.

In 2009, an epidemic of late blight occurred in the northeast U.S., due in part to a lack of public understanding of the symptoms and impact of the pathogen. Tomato plantlets infected with late blight were sold at big box stores across the U.S. at discounted prices. This caused blight to spread across several states. The disease spread from home gardens to tomato and potato crops and caused huge losses to the industry. The symptoms were somewhat different than those caused by the predominant strain US-8.

As part of the effort to increase communication between growers, home gardeners, extension agents and the research community, scientists in Canada and the U.S. developed programs to aid in tracking outbreaks of late blight and identifying the causal strains. Knowing the strain would help to identify the best management practices to halt the progress of the disease and protect the crop. In Canada, the National Late Blight Working Group was established by the Canadian Potato Council of the Canadian Horticultural Council. In the U.S., a national project on tomato and potato late blight was established under the name USAblight. The objective of these national programs is to provide both a tool for potato and tomato growers for managing late blight, and a way to better understand the movements and impact of this disease.

Following the 2009 epidemic in the U.S., new strains were identified in potato and tomato plants, including US-22, US-23 and US-24. Due to global movement of plant materials, these strains spread throughout Canada and the U.S. Some of these strains were short lived, and the predominant late blight strain in North America now is US-23, which is equally infective in potatoes and tomatoes. This strain of late blight is aggressive on tomato foliage and fruit. However, it is less aggressive on potato foliage, but aggressive on potato tubers.

Good late blight management practices include disease prevention, sanitation, cultural practices, field monitoring, an effective fungicide spray program and postharvest protection.

Disease prevention can be achieved by planting healthy seeds or plants. This helps give a good start to a healthy crop. Tools are now available to test for the presence of late blight in seed potatoes and plants. Threshold for late blight in seed potatoes or plants is zero. Once an infected seed or plant is planted, it can cause the disease to spread throughout the field and neighbouring fields. In some jurisdictions in Canada and the U.S., cull pile disposal programs are now legislated and growers are given a deadline by which they have to dispose of their cull potatoes in a proper way. Cull potatoes can grow to produce foliage that may become infected with late blight. Once infected, they become a source of inoculum. These piles are usually forgotten and never receive protection. A fly-over is conducted to monitor potato farms and look for any forgotten piles.

For a disease to occur, three pillars have to coexist: host (crop), pathogen and environmental conditions that are conducive for disease development. New technologies are now available to monitor weather conditions and predict when they become conducive to disease development. Weather stations can be placed on farms and collect local data that can then be analyzed and used in prediction models. Growers receive alerts when the weather becomes conducive to late blight. With regards to the presence of the pathogen, a new tool is now available to the industry: spore traps can be used to check the air for the presence of late blight spores in a particular area. Once spores are caught, growers are alerted. If the weather is conducive and the spores were found, growers will then apply proper management tools.

Several new chemistries are available in the market to combat late blight. A preventive spray program is always recommended. Effective control by fungicides requires good coverage of the foliage, proper rates and timing of applications. Generally, fungicides are most effective in the early stages of infections before symptoms appear. However, no fungicide can cure an established infection. Fungicides against late blight are essentially protectants and not particularly persistent. They must be used to protect plants as prophylactic sprays in routine programs, in an overall strategy designed to prevent the disease infecting the crop.

Initially, contact fungicides dominated the market. This was followed by the introduction of translaminar and systemic fungicides. Contact fungicides retain on the surface of the plant where they are applied and only protect the plant where the spray is deposited or been subsequently redistributed by moisture. They are not taken into the plant and therefore are vulnerable to erosion by wind, rain and degradation by sunlight. They do not protect new plant growth formed after the spray has been applied. These fungicides have no effect against already established late blight infections.

Translaminar fungicides are absorbed by the leaves and show limited redistribution from upper-sprayed surface to lower unsprayed surface. They are generally more rainfast than contact fungicides, but do not move within the plant to protect the new growth.

Systemic fungicides are absorbed into plant tissue and may offer some after-infection activity. Very few fungicides are truly systemic (i.e., move freely throughout the plant); however, some are upwardly systemic (i.e., move only upward in the plant through xylem tissue), and some are locally systemic (i.e., move into treated leaves and redistribute to some degree within the treated area of the plant).

Postharvest protection of potato tubers against late blight and other diseases is very important. New weather monitoring technologies are now available to help growers decide when to harvest their corp. Harvest advisories provide information on soil moisture and temperature and whether they are suitable for harvest. Harvesting when the soil is too cold/hot/dry/moist might affect the quality of harvested potatoes and their vulnerability to infection with diseases.

Harvested potato tubers are living organisms and hence interact with the surrounding environment. To maintain potato quality during storage, the storage environment must be adjusted to minimize tuber deterioration. Temperature, humidity and air movement can always affect the keeping quality of stored potatoes. When potatoes are not properly stored, tuber losses due to fungal and bacterial infections can be high. Various tools and sensors are available to growers to use in monitoring their storages.

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