What’s coming and what can you do?

This tomato leaf is infected with late blight. Note the dark color of the affected area and the presence of white fungal growth

If you grow tomatoes or potatoes, take heed. This is shaping up to be a year in which late blight causes a lot of damage in the Northeast. Late blight is a fungal disease caused by Phytophthora infestans, and it’s somewhat famous as the cause of the Irish potato famine of the mid-1800s. This disease doesn’t pose the threat of famine today, since we don’t depend on one or two local crops for our food, but it can cause economic hardship for growers that have potatoes and tomatoes in their crop mix.

Fortunately, extension personnel have been getting the word out about preventative materials that can be applied to protect these crops. The bad news is that if the pressure is high enough, even frequent spraying may not be sufficient, and for organic growers there are only a few materials, of limited efficacy, to choose from.

Why this year?

It takes three things for a plant disease to prosper: a susceptible host, a source of infection (inoculum) and the necessary environmental conditions. Early in this growing season much of the Northeast has had all three.

As a result, the disease showed up much sooner than it typically does. Also, in most years, only certain areas of the region are affected, but this year the disease has been quite widespread. Part of the problem seems to be that late blight-infected tomato plants were distributed to and sold at a number of home garden centers, setting up home gardens across the region as sources of inoculum to spread the disease.

Late blight spores can be carried long distances by wind currents, so it’s important to keep plants protected when conditions are favorable for infection, and to scout crops on a daily basis for signs of infection.

Late blight has the potential to kill tomato and potato plants; not a pretty sight.
Late blight can also cause the dark lesions on the stems of tomato and potato.

The symptoms that develop on tomato and potato leaves, stems and fruit are dramatic, and they’re obvious to the naked eye. The infected areas on leaves appear dark and water-soaked and are rather large, varying in size from a nickel up to a quarter. Symptoms often begin at leaf tips or edges. The disease can spread rapidly when foliage has been exposed to watering, rainfall or overnight dews and the weather is on the cool side. If the weather gets hot and dry, the disease won’t spread as fast, and affected areas may appear lime-green or beige in color. When there’s enough moisture around, you’ll see white fungal growth (mycelium) associated with the infected areas. Brown to almost black lesions may also appear on infected stems, and the same lesions will develop on fruit, either directly on the infected plants or a few days after harvest.

An excellent set of photos showing many aspects of late blight symptoms can be found at www.hort.cornell.edu/lateblight.

Primary hosts for the disease are potatoes and tomatoes, but weeds in the Solanaceae family, such as bittersweet black nightshade, may also become infected. Eggplant does not appear to be very susceptible and peppers even less so. Petunias can also host late blight. At this time, it appears that the strain of late blight we have in the Northeast this year is more aggressive on tomato than potato crops.

Before you sound the alarm, be sure that what you see is indeed late blight. There are several other common and less serious diseases of tomato and potato that are found every year on Northeast farms, such as early blight and Septoria leaf spot. If the infected area has a yellow border and is occurring on the bottom of the plant, it’s probably due to one of these diseases, not late blight. Septoria lesions are small spots, the size of flea beetle feeding, and early blight lesions have a characteristic bulls-eye pattern of concentric rings in the affected area that you can see if you look closely.

If you have doubts about whether or not your crops have late blight, contact your local agricultural extension agent to find out where to send samples of diseased plants for a positive identification of the cause.

Organic sprays

The best option for protecting crops organically is fixed copper. The better you can keep copper covering the plant,, Inc.luding new growth, the better the chance of avoiding late blight. Wet weather not only promotes the disease, but makes it difficult to spray; do your best to follow a five-day application schedule. Waiting longer than that will leave new growth unprotected and allow earlier applications to wash off, reducing effectiveness.

Materials approved for certified organic use are on the Organic Materials Review Institute list (www.omri.org/crops_category.pdf). Two approved copper materials are Champ WG (NuFarm Americas, Inc.) and Nu Cop 50 WP (Albaugh, Inc.). Adding a biological fungicide to your spray schedule in addition to copper might help suppress late blight, but these would probably not be effective on their own; these options, Inc.lude Serenade or Sonata.

When spraying these fungicides, remember they only protect healthy tissue; infected leaves or stems cannot be saved. Good coverage of all the foliage is critical, and repeat applications are needed to protect new growth from infection. Always read the pesticide label and follow the instructions carefully.

Tomato fruits may show late blight symptoms on the plant, or they may develop after harvest.

If late blight is found on a few plants, carefully remove them and place in plastic bags, along with surrounding plants that may not yet appear to be infected. Dispose of these so spores will not spread. For somewhat larger areas, a dark tarp can be placed over the plants to kill them-and hopefully the spores, too.

If the disease gets out of hand, it’s important to promptly destroy the crop to limit its spread. Fields should be thoroughly disked in to allow plants and spores to decompose. Wash equipment off with a power washer before moving to other areas with susceptible crops.

Late blight is an obligate parasite and needs living tissue in order to survive. Luckily, late blight is not seedborne, so tomato plants started from seed locally should be free of the disease, at least initially. Potatoes can carry the disease on tubers, so buying certified seed is important, as is destroying any cull piles promptly. For more information on late blight, visit http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm or http://ohioline.osu.edu/hyg-fact/3000/3102.html.

The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.