Fruit Pest News
Volume 8, No. 19 August 8, 2007
An online newsletter whose goal is to provide all interested persons with timely information on diseases and insects of commercial fruit and vegetable crops in Tennessee.
Text appearing in blue or red can be clicked to link to other web sites. Be aware that much of the linked information is produced in other states and may not be applicable to Tennessee.
In This Issue:
1. Pumpkin Fruit Rots
2. Cucurbit Downy Mildew
3. Cucumber Beetle Feeding on Cucurbit Fruit
4. Southern Blight of Vegetables
5. Black Knot of Plum and Cherry
1. Pumpkin Fruit Rots
Kind of early to start thinking about fruit rots, huh? Actually, this is the best time. The best defense against invasion of the fruit by rotting organisms is fruit rind tissue with strong cell walls. To obtain such "tough" fruit, the plant must be kept healthy during the growing season. Healthy leaves provide carbohydrates to the fruit, which strengthens the cell walls.
Provide adequate calcium, avoid excessive nitrogen, and irrigate during dry periods. Follow a recommended spray program to control diseases and insects. Controlling foliar diseases helps maintain healthy leaves and reduces the populations of organisms that can infect the fruit. No fungicide is very effective in reducing populations of the Fusarium fungus, our most common cause of fruit rots. However, Fusarium fruit rot can be reduced with these crop management steps.
Crop rotation is important in reducing the populations of fruit-rotting organisms such as anthracnose and bacterial spot. Two years in a non-cucurbit crop should be allowed between pumpkin crops. Small fruited pumpkins with the hard shell gene, such as Li’l Ironsides, Iron Man, and Cannon Ball, have a lower incidence of fruit rot. (SB)
2. Cucurbit Downy Mildew
Downy mildew seems to be keeping its distance from the Mid-South, where the drought is epi-centered. But new reports continue to come in. The distribution can be found at this site. Don't let the hot, dry weather fool you. One night of heavy dew is all that is needed for infection, if spores are present, although rainfall aids spore dispersal after the initial infection takes place.
Apply chlorothalonil or mancozeb every 7 to 10 days, for protection. Be prepared to add a more specialized fungicide such as Tanos, Previcur Flex, or Ranman if downy mildew occurs in your vicinity. You need to choose two specialized fungicides, because they need to be alternated, to discourage resistance development. (SB)
3. Cucumber Beetle Feeding on Cucurbit Fruit
This time of year, the fruit of cucurbits are susceptible
to chewing by cucumber beetles. This produces scarring of the fruit, which can particularly
be a problem on pumpkins. Inspect fruit (especially the underside) from now until
harvest for the first signs of feeding damage and treat then to prevent additional
scarring. Also, check for squash bugs that can be found on fruit, vines and foliage.
When treating for these pests, be aware of the preharvest intervals for the insecticide
that you choose. (FH)
4. Southern Blight of Vegetables
Southern blight, also known as southern stem blight, is a serious disease of many vegetable crops, causing an almost certain death of affected plants. It is caused by the soil-borne fungus, Sclerotium rolfsii, and attacks a number of vegetable crops including bean, cantaloupe, carrot, potato, pepper, tomato, eggplant, sweetpotato, tomato, watermelon and others.
Southern blight is one of the most common causes of a sudden wilting and death of a plant. Mild yellowing of the leaves may occur prior to wilting. Under humid conditions, a thin, white, fan-shaped mold forms on affected stem tissues and adjoining surface soil. Even under dry conditions, at least a trace of the white mold should be evident on the stem surface. Soon after mold formation, seed-like bodies (sclerotia) develop in the mold. The sclerotia begin white, turning tan, then bronze. When the plant is pulled up, a brown, dry rot of the lower stem and upper roots is apparent. In vegetables in which the fruit contact the ground, such as pumpkin and cantaloupe, the fruit are rotted, beginning with the side of the fruit in contact with the soil. On the surface of the edible roots of sweetpotatoes are 1/4 to ½ inch circular, sunken, dark gray spots.
The fungus overwinters as sclerotia in the soil and in plant debris. A characteristic of the fungus is that it is generally restricted to the upper 2 or 3 inches of soil and will not survive at greater depths. The fungus is more active in hot, wet weather, and it requires the presence of undecomposed plant residue to initiate infection. S. rolfsii is more active under acidic soil conditions. The fungus does not have an air-borne spore, so all infections result from contact of the plant tissue with soil. It is spread when infested soil particles are moved, as with cultivation. The fungal body is so strong that it is capable of growing across the soil surface to reach a plant, if old plant debris is available.
Control
5. Black Knot of Plum and Cherry
Black
knot, caused by the fungus Apiosporina morbosa, is a common and destructive
disease of plum and cherry. Losses in commercial orchards are usually not extensive
except where the disease has become established. Residential plantings of edible
or ornamental plum or cherry are commonly plagued with black knot. When left uncontrolled,
purple leaf plum trees sometimes have to be removed.
Black knot affects only
woody parts of the tree, mostly twigs, and is characterized by elongated swellings.
When first formed, knots are olive green and corky in texture,
and may be covered by a mold. Later, they turn black
and become hard and brittle.
The fungus, living in the knots, produces spores
that are discharged to other twigs, where they germinate and produce new infections.
Young growth is very susceptible to infection. However, older branches can also be
infected, especially if the bark has been damaged. Most infections occur during bloom,
petal fall, and shortly afterwards. Some additional infections occur throughout active
shoot elongation. Swelling of the twigs does not occur until autumn or the following
spring. The knots develop rapidly the second summer.
Prune out all knots,
cutting the stem a few inches below the knot. All pruned material should be removed
from the area and destroyed or buried. Chemical protection against black knot can
be afforded with captan, chlorothalonil, or Topsin M. Chlorothalonil cannot
be used after shuck split. A fungicide spray may be needed throughout active shoot
elongation. During the critical periods mentioned above, an application may be needed
every week.
Among the most susceptible varieties of plum are Damson, Stanley,
Bluefree, and Shropshire. Moderately susceptible are Methley, Milton, Fellenberg,
Bradshaw, and Early Italian. Less susceptible are Formosa, Shiro, and Santa Rosa.
President is highly resistant. (SB)
The Fruit Pest News URL is: http://web.utk.edu/~extepp/fpn/fpn.htm
Contacts:
Steve Bost, Professor and Extension Plant Pathologist
Frank Hale, Professor and Extension Entomologist
Both authors available at:
615-832-6802
fax 615-781-2568
Plant and Pest Diagnostic Center
5201 Marchant Drive
Nashville, TN 37211
Copyright 2007 The University of Tennessee. All rights reserved. This document may be reproduced and distributed for nonprofit educational purposes provided that credit is given to University of Tennessee Extension.
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