Preventing, identifying and managing disease in vegetable crops

If the weather this year is again rainy and cool at the start of the growing season, Dr. Cheryl Smith has some advice: “Look for a lot of happy pathogens.” Pathogens, she says, are usually more of a concern during wet seasons than insect pests. Smith, extension professor and plant health specialist with the University of New Hampshire Cooperative Extension, knows just how rainy weather can favor the development of diseases and how continued wet weather can contribute to additional disease spread. “One of the simplest and most obvious ways to help prevent the start of diseases is through good cultural practices and sanitation, such as removing the prior season’s infected crop debris,” she says.

Spores of early blight are dry and can be spread by the wind.
In wet conditions, spores can germinate in only two hours.
To help prevent bacterial canker, use certified disease-free seed. The disease can be splash-dispersed, but usually begins on seeds or transplants.

Tomato tribulations

Again this year, Septoria leaf spot and early blight will likely be problems. “Pretty much guaranteed. Our weather is very favorable for both,” she says. Both diseases first appear on the lower leaves.

• Septoria leaf spot and early blight

Septoria leaf spot is one of the most destructive of all tomato diseases. Lots of little (about 1/8 inch) spots show up on the leaves, stems and petioles but not on the fruit itself. Between seasons, Septoria leaf spot survives on crop debris or weed hosts, and on both wooden and metal stakes, cages and trellises.

Early blight, also hits leaves and stems, but it also attacks the tomato fruit. Because the spores of early blight are dry, they can be spread by the wind. “In wet conditions, spores can germinate in as little as two hours,” says Smith. The spots of early blight are up to .25 inch, and they often have a yellow border. Within the spots, concentric rings are often visible.

To help manage both Septoria leaf spot and early blight, tomato crops should be rotated, ideally on a two or three-year cycle, but not with eggplants or potatoes. Weeds, especially those in the nightshade family, should be controlled, and after harvest all debris should be plowed under. Unless compost is really cooking, do not include plants infected with Septoria leaf spot or early blight in compost piles. If tomatoes are watered with a sprinkler, be sure to water first thing in the morning so plants are not left wet all night. Get plants off the ground and either trellis, stake or cage them, and mulch to create a barrier to spores that might otherwise splash up from the soil. Fungicides are often necessary to manage both diseases.

• Bacterial canker

Bacterial canker can affect plants at any stage of growth. Seedlings may be infected, but not show symptoms until they are transplanted. Bacterial canker produces a wide range of symptoms. Look for leaf spots and dark browning on leaf margins and sunken areas on stems. Discoloration can also be seen on the insides of stems. Although this pathogen can be splash-dispersed, it usually begins on seeds or transplants. “In 2009, I saw more bacterial canker than I have seen any other year,” says Smith.

Late blight, here on a tomato leaf, will not overwinter on tomato debris, but can remain on potato tubers left in the ground or in compost piles.

First and foremost, use certified disease-free seed. Many seed suppliers will, on request, do a hot water seed treatment. Temperature of the hot water varies with type of seed. Tomato seeds need 25 minutes in 122-degree water. If treating at home, keep seeds stirred so that none fall to the bottom and overheat. Small cheesecloth packets work well to hold the seeds. The process will result in some loss of seed viability, so extra seed should be treated.

Stakes, cages and trellises, both wood and metal, that were used previously should be sanitized. Soak them in a 10 percent bleach solution for 20 to 30 minutes. “Be sure to mix up a fresh bleach solution every few hours,” says Smith. “Otherwise, you are just washing your stuff in dirty water.”

To keep from transferring spores and bacteria, avoid working in fields when plants are wet. This is particularly important when bacterial diseases may be present. If areas of fields are infected, work them last.

• Late blight

“Since late blight will not overwinter on tomato debris, we are hoping not to see in 2010 what we saw in 2009,” says Smith. “Although it can’t overwinter on last year’s infected tomatoes, late blight can remain in infected potato tubers left in the ground or in compost piles that did not freeze.”

Late blight on tomatoes.

Symptoms include large, olive green to brown, irregularly shaped and water-soaked spots on leaves. Under wet or very humid conditions, a slightly fuzzy white fungal growth may be visible on the underside of the leaf. Brown to blackish irregularly shaped lesions also develop on stems. Firm brown spots develop on tomato fruit and infected fruit often looks bumpy. For photos of late blight, see http://extension.unh.edu/Agric/lateblight.htm.

It is critical that all volunteer potato plants that sprout from last year’s tubers, including those in compost piles, be destroyed. Scout tomato plants for any symptoms of late blight. If late blight is suspected, have it confirmed by a diagnostic lab. Look for information and alerts at http://extension.unh.edu (click on agriculture, and check for late blight updates.) “Be a good neighbor,” says Smith. “Tell other growers if you find late blight so they can protect their crops.”

Cucurbit concerns

• Powdery mildew

As usual, expect to see this old foe.

Copper fungicides offer some protection against the spread of angular leaf spot, a bacterial disease transmitted by seed.

The best offense is a good defense: grow resistant varieties. Then, scout regularly, and be sure to check the undersides of leaves. Many fungicides, both organic and conventional, are available.

• Angular leaf spot

A bacterial disease transmitted by seed, angular leaf spot is so called because disease spots are confined within the angles created by the veins of leaves. If the weather is wet, expect to see Angular leaf spot on cucurbits, especially cucumbers and pumpkins.

Because angular leaf spot can be transmitted by seed, hot water seed treatment is suggested. A two-year rotation out of cucurbits is recommended. Copper fungicides offer some protection against spread of this disease.

• Cucurbit downy mildew

In cucurbits, downy mildew is a potentially devastating disease. It can be recognized by angular yellow to brown spots on the upper surface of the leaves, and a brown to purplish fuzzy fungal growth is often visible on the underside of the lesion when humidity is high. While downy mildew does not overwinter in the Northeast, it does get blown in, sweeping northward up the coast from the Gulf. It may also blow southward along the coast from Canada.

Plant resistant varieties. Increasing plant spacing and avoiding overhead irrigation will also help reduce the disease. Monitor www.ces.ncsu.edu/depts/pp/cucurbit/ for timely information on the occurrence of cucurbit downy mildew and forecasts for the movement of the fungus across the North American continent. Growers are encouraged to submit reports to the above site if they discover this “destructive and fast-moving plant disease epidemic.”

Plectosporium

Plectosporium can be managed by broad-spectrum fungicides. Disease-resistant varieties of spaghetti squash are not available.

In summer squash and pumpkins, white diamond-shaped lesions smaller than .25 inch appear on vines, stems and on the pumpkin handles. Lesions can coalesce, causing large areas of the stems, handles and fruit to turn white. Zucchini is particularly susceptible, and infected fruit is often unmarketable. Plectosporium has been on the increase over the last several seasons.

Disease-resistant varieties are not available. Plant in areas where there is good air circulation, avoiding low areas. Scout for Plectosporium at the same time you look for powdery mildew. Any of the broad-spectrum fungicides such as chlorothalonil (trade name Bravo) work well to manage Plectosporium. Crops should be rotated on a two to three-year cycle.

• Phytophthora blight

A blight of peppers and pumpkins in particular, Phytophthora can also hit eggplants and occasionally tomatoes. The fungus can attack all parts of plants, and symptoms vary depending on the stage of growth and the part of the plant infected. Young plants are killed quickly, while older plants may show wilting. Infected fruit has dark water-soaked areas that often are covered with white fungal growth and spores, especially in moist conditions.

The fungus can survive both in soil and on crop debris. “If Phytophthora blight has been present in fields, expect to see it again,” says Smith. Plant in well-drained fields, avoiding those with a history of Phytophthora. The higher the organic matter in the fields, the better. Use only drip irrigation. Crop rotation is critical, and a cycle of three or more years is ideal. Particularly with this disease, do not rotate cucurbits with solanaceous crops such as eggplant, pepper, tomato or potato. Fungicides are effective, so consult the 2010 – 2011 New England Vegetable Management Guide for suggestions. Rotate fungicide classes to avoid developing fungicide resistance.

• Bacterial leaf spot

Bacterial leaf spot may, as usual, appear on non-resistant varieties of peppers. Plant resistant varieties.

• Sclerotinia root rot (also known as white mold)

A very fluffy white mold, Sclerotinia root rot causes both stem canker and root rot. Last year saw lots of Sclerotinia stem rot on tomatoes and peppers, but this disease has a wide host range, which can also include beans, squash, pumpkins and carrots. The fungus can remain in the soil for 10 to 15 years or more.

If the Northeast again has wet spring weather, Dr. Cheryl Smith, extension professor and plant health specialist with the University of New Hampshire Cooperative Extension has some advice: "Look for a lot of happy pathogens."
Photo by Kathleen Hatt.

Leave any field unplanted in which Sclerotinia root rot was present last year. “It may be best to leave fields unplanted for at least two years or perhaps rotate them with grains,” says Smith. Plant potentially affected crops in areas with adequate air circulation, and pay careful attention to sanitation. Also avoid overhead irrigation. Provide adequate potassium and avoid excess nitrogen. Contans WG, an OMRI-approved biological control fungus that parasitizes the sclerotia of Sclerotinia, is one option for organic producers.

More help

For disease management recommendations, see the 2010-2011 New England Vegetable Management Guide. The guide, which also includes a section on biorational disease control products, including those approved for organic use, may be found online at www.nevegetable.org.

Early identification and careful attention to rotation and sanitation are crucial to the management of all these potentially devastating diseases. “Start clean, rotate and scout, scout, scout,” says Smith.

For Further Information

University of New Hampshire Plant Diagnostic Lab
Diagnostic form to submit with sample:
http://extension.unh.edu/Agric/Docs/PDLformfinal.pdf
$15 fee for most samples.

Northeast Plant Diagnostic Network
List of extension-affiliated diagnostic labs in member states
www.NEPDN.org

2010 – 2011 New England Vegetable Management Guide
Order from any extension service in New England. Includes instructions for hot water seed treatment and the requisite temperature by type of seed.

Kathleen Hatt is a freelance writer and a frequent contributor to Growing. She resides in Henniker, N.H.