Small fruits, tree fruits and more
Rainy and cool weather at the start of the Northeast growing season could once again trigger the appearance of a lot of plant pathogens. Dr. Cheryl Smith, extension professor and specialist in plant health at the University of New Hampshire, explains diseases that moist weather conditions may foster in small fruits and tree fruits and in garlic and basil and suggests ways of dealing with them.
Raspberries and strawberries
Botrytis gray mold/fruit rot, Botrytis cinerea
Cool and wet weather favors what is perhaps the world’s most ubiquitous pathogen. A common cause of fruit rot, botrytis gray mold spores are dispersed in spring by wind, rain or irrigation splash. High relative humidity favors the development of disease, and spores landing on wet fruit can germinate quickly. As usual, look for botrytis to be a problem during bloom and also on ripening, mature and harvested fruit. Infections that occur during bloom may not show visible symptoms until the fruits begin to ripen.
As always, good air circulation and good sanitation are key to managing this disease. Thinning raspberries to five to seven canes per linear foot of row and maintaining strawberry rows to no wider than 2 feet should help keep relative humidity low and promote rapid drying. Weed control is also critical. Many fungicides are available and may be applied both to early bloom and at full bloom. (See the New England Small Fruit Pest Management Guide.) Be sure to rotate classes of fungicides to prevent the development of fungicide resistance.
Especially in raspberries, but in other brambles as well, be on the lookout for cane diseases such as:
• Spur blight, Didymella applanata
Although wet weather in early spring contributes to development of spur blight, infections do not become visible until mid to late summer when brown to purple blotches appear around buds on canes. Infections occur more frequently on red raspberries than on black raspberries. To help control spur blight, avoid both overcrowding and excess nitrogen.
• Anthracnose, Elsinoe veneta
Wet springs contribute to development of anthracnose. Scattered purple spots appear on canes, expanding to about 1/8 inch. The purple spots become sunken in the center and then turn gray with a purple border. Spots can grow together to form large, rough-looking areas on the canes.
Maintain good air circulation with an open planting, no more than five to seven canes per linear foot. Good weed control is also essential to promote rapid drying. Wild brambles on the edges of fields are a good source of pathogens and should be removed. A late dormant application of lime sulfur will help reduce disease spread. To avoid plant injury, lime sulfur should not be applied once the new growth has reached .5-inch. For fungicides effective on cane diseases, see the New England Small Fruit Pest Management Guide.
Phomopsis, tip blight, canker and fruit rot
“We’re seeing a lot more canker diseases on blueberries, and they can all look similar,” says Smith. Spread by rain splash, phomopsis causes tip dieback. Affected tips closely resemble winterkill. Phomopsis can also cause fruit rot. The disease is favored by spring rains and is more common following frost or winter injury.
When planting, look for resistant varieties. In the late dormant period just before bud break, apply a lime sulfur spray. A second lime sulfur application in late October may help to reduce infections that occurred during the growing season. Avoid late summer fertilization. If phomopsis does occur, promptly prune out cankered stems and destroy them.
Troubles with garlic
“Cool weather and wet soil, and especially wet weather during harvest, provide ideal conditions for the growth and spread of fusarium rots,” says Smith. In the past four years, fusarium rots have become an increasing problem. Plants may not show symptoms in the field or at harvest, but will later rot in storage. Infected bulbs appear spongy. A light pink or reddish fungal growth may be visible on the clove scales and brown, sunken areas usually develop on the cloves themselves.
It is important that any garlic displaying symptoms of a fusarium rot be destroyed immediately. Do not keep seed from crops that have been infected with any rots. Even symptomless cloves can harbor the fungal pathogens. In wet years, it may be better to harvest the crop early rather than to wait for the weather to dry out. The longer the crop remains in cool, wet soils, the greater the chance of infection. Avoid exchanging seed with other growers. Since fungicides are of questionable effectiveness, prevention is the best defense. Buy new, certified seed, and rotate any affected field out of garlic for two to three years. The fusarium diseases occur primarily on Allium species, but several small grains may also be infected, so rotations with cereal crops should be avoided in fields with a history of fusarium rot.
Mildew on basil
Basil downy mildew
A seed-borne disease, basil downy mildew first appears as a slight yellowing between veins on upper leaf surfaces. The undersides of leaves exhibit a dusty-looking downy mildew fungus. This disease, which also occurs on coleus and salvia, was first seen in the U.S. in 2007.
Grow resistant varieties of basil or use certified or treated seed. If basil downy mildew does appear, “Destroy plants as soon as possible,” says Smith, “because this fungus spreads like crazy.” Basil downy mildew can spread by air and move from greenhouse to field. “It also appears to be the same fungus that causes downy mildew on coleus,” says Smith. OMRI-approved controls include Oxidate and Actinovate. Actinovate cannot be used in the greenhouse. On herbs in the greenhouse, phosphorus acid products like Pro-Phyt and K-Phite can be used. There are no effective fungicides for use on basil because it is an edible crop.
To manage all of these pathogens that are likely to recur this year and tend to be more prevalent in wet and cool conditions, Smith suggests scouting early and often and treating promptly to avoid further spread of disease. To confirm what pathogen you are dealing with and to make sure you are using the most appropriate management tools, Smith says, “When in doubt send it out.”
For further information:
New England Small Fruit Pest Management Guide, 2003-04. Order from University of Massachusetts Extension, Amherst, www.umass.edu/fruitadvisor/nesfpmg
2010–2011 New England Vegetable Management Guide, Order from any extension service in New England, e.g., UNH Extension, or from the University of Massachusetts Publications Office.
University of New Hampshire Plant Diagnostic Lab.
Download diagnostic form to submit with sample at http://extension.unh.edu/Agric/Docs/PDLformfinal.pdf.
There is a $15 fee for most samples submitted.
Kathleen Hatt is a freelance writer and a frequent contributor to Growing. She resides in Henniker, N.H.