MN/ND Late Blight Alert (pdf version)

Late Blight: A Plant Disease That Impacts the Community

Late blight is a potentially devastating plant disease that affects potatoes and tomatoes. Other related plants such as petunia and hairy nightshade also are affected. Don’t let the name fool you. This plant menace can show up any time during the growing season on a susceptible plant when the right weather conditions and the pathogen are present. The pathogen that causes late blight is windborne and can devastate yields quickly in the garden and commercial fields if left unchecked. As a result, late blight is a community problem that affects everyone. Gardeners and growers alike have a responsibility to do their parts to control it.

Late blight outbreak history

Late blight is caused by Phytophthora infestans, a pathogen in the “water molds” group of organisms. It is the same disease that destroyed potato crops in Ireland, leading to the Irish potato famine in the 1840s. This disease has occurred sporadically in North America for decades. In 2009 and 2010, the disease was found at epidemic levels in the north-central U.S. and parts of Canada, devastating tomatoes and potatoes in gardens and commercial settings.

Why did late blight reappear?

Several strains of the pathogen occur, and in the past, most strains in the central North American region infected only tomatoes or potatoes. New strains of the pathogen that are able to infect both potatoes and tomatoes have been identified during the recent epidemics. Evidence suggests that the source of some of the new strains was infected tomato seedlings from production facilities in the South that were distributed to big-box retail stores across North America. Widespread cool, wet weather during the 2009 and 2010 growing seasons was particularly conducive to disease development, and the disease progressed rapidly. Many gardeners and small producers were unprepared for the recent epidemics and did not recognize the disease until too late.

Weather conditions that promote late blight development

The late blight pathogen requires cool to moderate temperatures and free moisture from dew, rain or overhead sprinklers. Temperatures between about 59 and 80 F allow the disease to progress rapidly, but the pathogen is active at a wide range of temperatures. Wind can carry the pathogen long distances.

Implications

The combination of several factors, including susceptible hosts (plants that can become infected with the late blight pathogen), new strains of the pathogen, inoculum buildup from recent epidemics and ideal weather suitable for disease development provide conditions for a “perfect storm” of annual late blight epidemics.

Late blight symptoms

All above-ground plant parts of tomatoes and potatoes are susceptible to late blight. On leaflets, late blight symptoms begin as small, water-soaked lesions that quickly expand to form large gray, pale green or dark brown blotches on leaves, petioles and stems (Figure 1). Under moist conditions, a thin layer of delicate whitish-gray or translucent moldlike growth will form on the undersides of leaves (Figure 1A). Sometimes, a narrow yellow margin forms around the lesion perimeter (Figure 1B). Stem lesions (Figure 1C) and leaf blotches will expand to encompass entire leaves, leaving them brown, shriveled and dead. At this point, foliar symptoms can resemble those associated with frost damage.

Figure 1A. A late-blight lesion on a potato leaflet, as seen from the underside of the leaf. Note the thin, dellicate white growth of the pathogen on the underside of the leaf along the perimeter of the lesion. Photo by NDSU.

Figure 1B. A late-blight infected potato leaf, as seen from the upper sid of the leaf. Sometimes, a distinct yellowish or light green margin surrounds the blotch. Photo by NDSU.

 Figure 1C. A lesion on the stem of a potato plant. This lesion can quickly expand and kill the adjacent leaves. Photo by NDSU.

On tomato fruit, initial blemishes and blotches appear somewhat greasy and are diffusely brownish or greenish gray (Figure 2). Affected fruits usually retain their normal firmness initially. The blemishes can expand to cover the entire fruit and, through time, the fruit will deteriorate from secondary soft rot organisms. The delicate thin layer of mold growth of the pathogen often is associated with fruit blemishes.

Figure 2. Tomato fruit with diffuse late blight lesion. Fruits are initially still firm; over time, secondary soft rot organisms will likely move in and cause mushy decay. Photo by Kasia Kinzer, NDSU.

The pathogen also can infect potato tubers. Affected tubers may have dry, reddish, granular lesions beneath the tuber skin (Figure 3). Early tuber symptoms often do not extend deeply into the tuber flesh, and they usually are more visible when a thin layer of the peel is removed. Affected portions can provide an entry point for secondary soft rot organisms that rot the tuber. Potato tubers infected with late blight often decay in storage due to secondary invaders.

Figure 3. Late-blight infected potato tuber. Initially, tuber infections are hard to see unless the skin is peeled slightly away to reveal the reddish-brown, granular, underlying infected flesh. As in tomato fruit, the tubers may be initially firm, but over time, secondary soft rot pathogens will likely move in and decay the tuber. Photo by NDSU.

Minimize the risk of late blight

No late blight-resistant potato varieties are available. Several varieties of tomatoes with late blight resistance are available through seed catalogs. However, as new strains of the late blight pathogen emerge, resistance levels in these tomato varieties may vary. As a result, growers should rely on other actions to minimize the late blight risk.

The following strategies do not guarantee disease prevention, but they may help minimize the risk of introducing late blight into the planting, slow the development of late blight and reduce the risk of producing infective parts of the pathogen that could threaten other small and large plantings.

• Start with healthy tubers and plants. Use certified potato seed tubers. Check them visually to make sure they are healthy before planting them. Start tomato and petunia plants from seed or select healthy seedlings from a local producer. Inspect tomato plants carefully, and do not purchase them if late blight symptoms are present.

• Improve growing conditions. Keep foliage as dry as possible. Avoid watering with overhead sprinklers because the resulting water splash maintains leaf wetness and can move the pathogen from infected to healthy plants. To allow foliage to dry more quickly, avoid planting densely to help improve airflow between plants. Stake tomatoes to increase air movement around leaves and fruit. Mulch with plastic or organic mulch such as straw or wood chips.

• Remove and destroy affected plants and weeds. In the spring before planting, destroy any potato tubers, tomatoes and volunteer weed hosts such as hairy nightshade that may have survived the winter. Vigilantly monitor for disease symptoms in the planting, starting early in the season . If plants with symptoms are found, they should be removed and destroyed immediately. Place them in a plastic bag and throw them away. Or, cover affected plants with a tarp to trap heat to hasten the death of the plant and pathogen. For larger plantings, plowing or torching may be warranted. By the time symptoms are observed, adjacent plants are likely to be infected and should also be destroyed. These actions are important not only to protect healthy plants in the garden but also to help minimize the chances that the pathogen will spread to neighboring gardens or commercial potato fields. At the end of the season, clean the planting area by completely removing and destroying all host plant parts. Composting late blight-infested plant debris in the fall is not recommended.

• Protect plants early with fungicides. If late blight is in the area, early action with protectant fungicide applications is critical to minimize the potential of the pathogen to completely destroy the garden harvest and spread to other areas. Applications should be made before the disease symptoms appear in the planting and at least once a week as long as the weather is cool and wet.

• Organic growers have limited options. Some of the copper-based fungicides meet organic standards set by the Organic Materials Review Institute (OMRI). Organic growers are urged to check the OMRI database (www.omri.org) for accepted copper-based fungicides. Because copper is not as highly effective for late blight control as chlorothalonil, applications should be made before disease symptoms appear.

• Not all fungicides are effective against the late blight pathogen. Only use a product that is registered for use against late blight on edible crops.

• Read, understand and follow the fungicide label instructions before use.

Late blight is a community problem that threatens all potato and tomato growers. Everyone needs to do his or her part to prevent and control it.

Authors: Gary Secor, Kasia Kinzer, Viviana Rivera-Varas, NDSU Plant Pathology

Reviewed by Michelle Grabowski, University of Minnesota Extension, and James Bradeen, Department of Plant Pathology, University of Minnesota

DRAFT VERSION: 29 April 2011

Sponsored by:

Minnesota Department of Agriculture

North Dakota Department of Agriculture

North Dakota State Seed Department

North Dakota State University Department of Plant Pathology

North Dakota State University Extension Service

Northern Plains Potato Growers Association

University of Minnesota College of Food, Agricultural and Natural Resource Sciences Department of Plant Pathology

University of Minnesota Extension Service