Fruit Pest News

Volume 5, No. 22  September 21, 2004

An online newsletter whose goal is to update Extension agents and growers of commercial tree fruit and small fruit crops

on diseases and insects 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. Apple - Sanitation Practices for Reduction of Apple Scab Inoculum for the 2005 Growing Season

    2. Pear - Stony Pit (Dimpling)

    3. Vegetable Corner

        a. Pumpkin Fruit Rots

        b. Two New Fungicides for Cole Crops and Greens

1. Sanitation practices in Fall 2004 and/or Spring 2005 may reduce apple scab inoculum for the 2005 growing season.

By Alan Biggs, Plant Pathologist, West Virginia University.

It is possible to reduce the amount of overwintering apple scab in your orchard using two practical and inexpensive methods: spraying trees with a urea fertilizer solution at leaf-fall or spraying fallen leaves on the ground, and shredding fallen leaves with a flail mower.

Here’s how it’s done:

Urea application: A 5% solution of urea (46-0-0) in water may be applied to apple trees as leaves begin to fall in the autumn (42 lb. urea in 100 gal. water). This should be done as late as possible to prevent the urea from being translocated into the tree. However, it should be early enough to have most of the leaves still on the tree. Note that trees sprayed with urea may defoliate more quickly than unsprayed trees. Urea may also be sprayed on the leaves on the ground, after all the leaves have dropped. The ground spray can also be done in the spring.

Leaf shredding: Mulch leaves with a flail mower in the fall. Mowing in the spring before budbreak will also be helpful. The amount of leaf litter that you can shred up will depend on the type of mower, as well as the tree canopy and spacing. If you can get at the leaves in the weed-free strip under the trees, it will work better than just mowing between the rows. (Another option is to flail-mow and then apply urea as a ground spray to the "in row" area that could not be reached by the flail mower.)

Here’s how it works:

Urea inhibits the development of apple scab fruiting bodies on the fallen leaves. The high nitrogen content also helps the leaves to decompose much faster than normal. Leaves sprayed within a week of leaf fall have been shown to produce few or no apple scab spores in the spring. If the urea-sprayed leaves hang on the tree longer than a week, too much nitrogen will be translocated out of the leaves, and scab suppression will not work as well. A follow-up spray of leaf litter on the ground would help to improve control of the fungus. Leaf shredding also hastens the decomposition of the leaves.

Here’s how well it works:

R.T. Burchill, (East Malling Research Station, England) published a paper in 1968 describing experiments with urea. Burchill found that when apple plants were sprayed in autumn with 5% urea, followed by a second pre-budbreak application of 2% urea to leaf litter, ascospore production in the spring was significantly suppressed. Spraying in the fall was effective when leaf abscission occurred within a week of treatment. However, a single autumn application still allowed some ascospore production. In 2 orchard trials conducted by Burchill, urea treatments resulted in a reduction in the number of scab lesions on blossom spur leaves by 46 and 59%.

Bill MacHardy (University of New Hampshire) has done some recent work on evaluation of sanitation practices. In a 1994 article, he reported on effectiveness of spring and autumn leaf shredding in orchards using a flail mower, as well as effectiveness of urea application in both spring and fall:

· Spring leaf shredding reduced ascospore dose by 89% and foliar scab by 80%.

How does this fit into a disease management program?

Sanitation measures will help to reduce the amount of initial inoculum. This will make scab easier to control, but fungicide sprays will still be necessary during primary scab season.

An orchard that had a severe scab problem in 2004 will benefit from reduced disease pressure, making it more likely to obtain effective control with fungicides during primary scab season. And remember - good control during primary scab season means you won't need to continue spraying for the rest of the summer. Another benefit is that lowering the disease pressure will make it less likely that the fungus will develop resistance to fungicides.

2. Stony Pit (Dimpling) of Pear

Ever seen a pear with a severe case of dimples? Could be stony pit. This is a virus disease characterized by deep indentations and deformity of the fruit. The tissue at the base of the indentation (pit) is sclerotized (hard) and brown in color. The sclerotization makes the fruit difficult to peel. Pits in pear fruit produced by other causes (insects, cork spot, boron deficiency) are much more superficial and are not associated with the production of sclerenchyma cells.

Stony pit is transmitted by vegetative propagation. There is no documented evidence of insect transmission. The severity of symptoms varies from year to year and even from fruit to fruit on the same tree. The varieties Bosc, Comice, and Seckel have more severe symptoms than other varieties. (SB)

3. Vegetable Corner

a. Pumpkin Fruit Rots

Pumpkins, gourds, and winter squash are an important commodity for apple growers. Some apple orchardists grow pumpkins and sell them at their stands to complement their apple sales. Others purchase pumpkins for resale at their stands. In either case, post-harvest rots can be a problem. Rotten pumpkins don't sell, and you don't want them to rot in Mrs. Customer's house, either.

Most efforts to control post-harvest rots of pumpkins, gourds, and winter squash should have already taken place. Most of the rots that appear in storage actually began in the field, during the production of the fruit. However, there are some steps you can take to lessen the development of rots after harvest.

Pre-harvest control measures. Rots develop when disease-causing organisms, mostly fungi, enter the fruit surface. The best defense against this invasion 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. 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.

Post-harvest control measures. Damaged pumpkin skin allows easier entrance of the fruit-rotting pathogens. Handle fruit carefully to avoid injuries. It is helpful to wash the fruit after harvest and allow to dry. Some growers dip or spray the pumpkins in a 10 percent bleach solution. This practice can reduce infections in storage, but will not stop the development of infections that occurred in the field. Holding the pumpkins at 80-85 degrees F and 80-85% relative humidity for 7 to 10 days allows scratched areas to cure. After the curing period, the temperature and humidity must be lowered to the levels recommended for storage (50-55 degrees F and 50-70% relative humidity), to reduce the disease potential.

Store pumpkins in a dry, shaded area, out of contact with the soil (straw or hay works well). Avoid piling pumpkins, as this practice decreases air circulation around them. Check stored fruits regularly for rots, and discard affected ones. Storage life of pumpkins is typically two to three months. (SB)

b. Two New Fungicides for Greens and Cole Crops

Two fungicides have received federal registrations for use on heading cole crops and greens. Switch and Endura will assist with control of Alternaria leaf spot and powdery mildew on broccoli, Brussels sprouts, cabbage, cauliflower, kohlrabi, collard, kale, and mustard. Endura can also be applied to turnip greens, but it is not effective against our most common diseases of turnip greens, Cercospora leaf spot or downy mildew. Switch has a 7-day preharvest interval and Endura has a 14-day preharvest interval. Resistance management programs are recommended for each product. (SB)

The Fruit Pest News URL is: http://web.utk.edu/~extepp/fpn/fpn.htm

Contacts:

Steve Bost, Professor and Extension Plant Pathologist

scbost@utk.edu

Frank Hale, Professor and Extension Entomologist

fahale@utk.edu

Both authors available at:

615-832-6802

fax 615-781-2568

Plant and Pest Diagnostic Center

5201 Marchant Drive

Nashville, TN 37211