Be prepared! – risk factors for late blight in tomato and potato

EDIT: Today, a finding of late blight was confirmed on potato in Adams County, Wisconsin. If you are a potato or tomato grower in the Midwest, now is the time to make plans to manage late blight, if you have not already. This post contains brief management advice and links to more detailed information. An upcoming post will provide specific information on organic protectants and fungicides.

As most of you know, potato and tomato growers in the Upper Midwest have contended with serious late blight outbreaks in recent years, starting in 2009. For 2013, late blight has been reported on potato in Wisconsin, on potato and tomato in Florida, and on tomato in Tennessee, West Virginia, Louisiana and Maryland. In past years, we have seen late blight epidemics begin in the eastern states and move across the country to the Midwest. The cool, wet and windy conditions that are prevailing across much of the northern US favor the development of late blight, and make this a possible scenario again in 2013.

Late blight is caused by the fungus-like pathogen Phytophthora infestans – literally “infesting plant-destroyer” – an appropriate name for a pathogen that can destroy a field of potatoes or tomatoes in a few days of cool wet weather. P. infestans is a fungus-like organism – a member of the oomycete group, also known as water molds. The pathogen produces two types of asexual spores. Sporangia can be spread long distances by wind, and will also spread from plant to plant in water. Zoospores, which can swim, are produced in large numbers from sporangia when free water is present. Sexual spores called oospores are only produced when two different mating types are present. Oospores are thick-walled and can over-winter in soil. Since only one P. infestans mating type has been found in the Midwest in recent years, oospore overwintering has not been a concern; however, the pathogen can overwinter in potato tubers or plants that are protected and survive the winter.

Integrated and effective management of late blight requires action at many stages of the year. Some management strategies are:

Sanitation. Don’t compost late-blight infected tomato or potato plants, or potato tubers. Plants may survive in compost or cull piles through the winter, and produce spores in the spring. Also, cull any volunteer potato plants in your fields or gardens.

Exclusion. Obtain seed potatoes from certified clean sources, and plant only healthy-looking tomato seedlings. Since late blight is not spread through true seeds, you can also start your own tomatoes from seed. Choose late blight resistant varieties. This fact sheet (pdf link) on managing late blight in tomatoes includes an extensive list of tomato varieties with resistance to late blight.

Scouting. Get familiar with the symptoms of late blight, and inspect your tomato and potato crops frequently. The following sites have useful pictures and descriptions:

Learn the symptoms of other common leaf diseases – early blight (which affects potato and tomato) and Septoria leaf spot (which affects tomato) – to distinguish these from late blight symptoms. Pictures and descriptions can be found at the following sites:

In addition, it’s a great idea to check on the risk of late blight in your region using the U.S. Pest Tomato-Potato Late Blight Risk map. Many regions have their own forecasting systems for late blight and other crop diseases; in Wisconsin, this information is provided through the UW Vegetable Pathology website.

Protective sprays. If the late blight risk in your region is high, you may choose to apply a protective spray to your crop. Copper fungicides provide some protection against late blight, but they must be present on the leaves before spores arrive – they cannot cure an existing infection. Not all copper fungicides are approved for use in organic management, so check with your certifier or the OMRI list (http://www.omri.org/). Application of copper fungicides carries risks, especially to the eyes, so read the label and follow all safety precautions. Also, be aware that copper accumulates in the soil and will also kill beneficial fungi. As with many management decisions, the choice to spray or not is a trade-off between different goals.

Other fungicides registered for use in organic management include Serenade®, Sonata®, Oxidate®, Sporan®, and Sporatec®. The active ingredients of these sprays are biocontrol bacterial (Serenade and Sonata), hydrogen peroxide (Oxidate) and essential oils (Sporan and Sporatec). These sprays are much less effective against late blight than copper sprays, as shown in several replicated trials. However, anecdotal evidence suggests that alternating these sprays with copper can slow the progress of the disease. Again, if you choose to use these sprays, read the label and follow all safety precautions. Even “mild” sprays can have dangers to human health and the environment.

Destroy infected plants. Infected plants cannot be saved, and it is best to destroy them, so that they do not provide a source of spores that may infect healthy plants. The pathogen can only survive on living plant tissue. Small numbers of infected plants can be pulled up by the roots, double bagged and left in the sun, since this will rapidly heat and kill plant and pathogen alike. Larger numbers of infected plants can be piled in a sunny spot and covered with a dark tarp, to heat and kill the plants. If it is necessary to destroy a large stand of staked tomato plants, cut the plants at the base of the stem and leave in place until the plants are dead, when stakes, ties and plants can be removed. Propane flaming is another way to kill larger numbers of infected plants. Consider killing a border area around the infected patch, to limit spread to healthy plants. Dead plant debris can be shallowly incorporated in soil, but should not be deeply buried or composted due to the risk of some plants, with the pathogen, surviving and continuing to produce spores.

County extension agents and university specialists are a resource for you if you suspect late blight in your garden or farm – do not hesitate to send pictures and samples to your local agents if you need help. Hopefully we will all get through this season blight free!

Best regards,

Ruth

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Organic Potato Project

Department of Plant Pathology

Be prepared! – risk factors for late blight in tomato and potato