Fruit Pest News

In This Issue:

1. Blackberries: Cane Blight
2. Plum and Cherry: Black Knot
3. Apples: Phytophthora Crown Rot and Collar Rot

4. Apples: Reminders for Late-Summer Disease Control

5. Processed Fruit: Dealing with Ronilan-Treated Fruit

1. CANE BLIGHT OF BLACKBERRIES

Cane blight, caused by the fungus Leptosphaeria coniothyrium, can be a damaging disease of thorned and thornless blackberries and black raspberries in Tennessee. Although usually associated with wet growing seasons, cane blight can also be severe following droughty summers. Drought stress also enhances symptom development in previously infected canes.

Cane blight symptoms appear on fruiting canes, although infection takes place on primocanes. These infections occur through wounds caused by abrasion against spines and old cane stubs, or by pruning or insect feeding. Since primocanes grow vigorously, the infections are suppressed until the following spring, when the canes enter the reproductive phase. Dark brown to purplish cankers form on one side of the cane, causing the death of axillary buds or wilt of individual lateral shoots as it continues to enlarge. Cankers can girdle the cane, causing it to wilt and die. The epidermis of the canker often develops a light gray color, with a dark purple border. Dark gray patches of spores ooze onto the surface from small fruiting bodies. Infected canes commonly become cracked and brittle, and snap off easily.

The causal fungus overwinters on infected or dead canes. The following spring, spores are released and carried by splashing rain and wind to nearby primocanes. Under moist conditions, the spores germinate and the fungus penetrates wounds and enters the vascular tissue. Fungal spread lasts from early spring until late fall. Dead canes continue to produce conidia and remain a source of infection for several years.

Try to avoid unnecessary injuries, and make pruning cuts on primocanes close to a node, so that the cane will heal without leaving a dead stub. Apply irrigation (trickle, not sprinkler) to improve the resistance of the primocanes to infection. Remove wild brambles growing in the area, as they provide a source of spores of this and other diseases. After harvest, remove and destroy all old floricanes. Do this job in rain-free weather, since many injuries to the primocanes will occur. The same is true for pruning operations to the primocanes.

Benlate is effective against the cane blight fungus and may provide valuable control in plantings prone to this disease. There is a limitation to the number of times Benlate can be applied, so time the applications for maximum benefit. Certainly, chemical protection cannot be provided for the entire infection period, which is the entire growing season. Benlate applications would be most critical following cultural practices that cause injuries to the primocanes, such as pruning and post-harvest floricane removal. Benlate applications must be timed to agree with the label. Adding a spreader-sticker to the Benlate solution would be helpful. (SB)

2. 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 Benlate, 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)

3. PHYTOPHTHORA CROWN ROT AND COLLAR ROT OF APPLE

Crown rot, collar rot, and root rot in apple trees are caused by Phytophthora species (root rots can also be caused by other types of fungi). These three diseases can occur on one tree and can be caused by a single infection, but are given different names to designate the part of the tree affected. Collar rot occurs in the scion part of the tree. This disease has decreased due to changing variety preferences and to the practice of raising graft unions above the soil line at planting. Crown rot is a disease of the rootstock portion of the tree. It has become more important because of the increased use of susceptible rootstocks such as MM106 and M26. Phytophthora root rot is a disease of the root system away from the crown region. It may occur along with crown rot or may occur by itself.

In the spring, symptoms of Phytophthora include delayed bud break, leaf discoloration, and poor growth. Foliage is sparse, yellowish, and may develop an early purple discoloration in autumn. In dry weather, infected trees are the first to exhibit symptoms of water deficit.

The most obvious symptom is a partial or complete girdling of the trunk with reddish-brown, water-soaked areas of dead tissue at the base of roots where they attach to the trunk. The entire underground portion of the trunk is usually water-soaked and brown with the dead area extending upward to the graft union.

The fire blight bacterium can cause rootstock cankers on M26 and M9 that are almost indistinguishable from crown rot. Fire blight cankers form in July and August following early season blossom and/or shoot infections. Rapid decline of trees on these rootstocks in late summer may be caused by fire blight cankers below the graft union.

Avoid planting fruit trees in poorly drained locations, and do not allow depressions to form around the base of trees. Where drainage is questionable, use the less susceptible rootstocks such as seedling and M9. Moderately susceptible rootstocks are M7, M26, and MM111.

Ridomil and Aliette fungicides are helpful in control, but should not be solely relied upon. They should be used in conjunction with cultural practices. Because Phytophthora diseases occur sporadically, it is usually not economical to treat entire orchards on a regular, preventative basis. Rather, use them in problem areas. Also note that these fungicides seldom are effective in reviving trees once the crown has become infected and moderate symptoms of decline have appeared. Ridomil can be used in the fall and spring only, applied as a soil spray. Dipping the roots of nursery-grown trees into a solution of Aliette prior to planting may reduce, but not eliminate, Phytophthora inoculum on roots. Aliette can be used as a foliar spray to nonbearing trees. (SB)

4. REMINDERS FOR LATE-SUMMER DISEASE CONTROL IN APPLES

Shower activity is resuming as fruit are entering their period of highest susceptibility to rot diseases. According to the weather service, wet weather can be expected this week as the tropical storm arrives from the gulf. This would be a good time for a fungicide application, so that it will provide protection during the extended wet period.

Keep in mind the preharvest intervals (PHI) of the fungicides needed for disease control near and during harvest. Although Flint and Sovran have good residual activity (around 21 days) the PHI of Sovran (30 days) precludes its use near harvest. The 14-day PHI of Flint reduces the time it provides protection after harvest begins. Spraying near and during harvest may leave undesirable residues on the fruit, but is necessary where fruit rots have become a problem. Captan, thiram, and Topsin M have 0-day PHI's, and would be the products of choice in such cases. (SB)

5. DEALING WITH PROCESSED FRUIT TREATED WITH RONILAN

As you may already be aware, the tolerance for vinclozolin (Ronilan) fungicide on strawberries and stone fruits was revoked, and the last date for use of existing stocks on these crops was January 30, 2000.  To deal with the processed foods that may remain in the channels of trade after that date, the Food and Drug Administration has issued a draft notice of guidance to industry. According to the document, FDA may waive seizure due to vinclozolin residues if the responsible party can show the food was packed or processed before July 1, 2000. If you would like details, you can view the document at http://www.cfsan.fda.gov/~ dms/vincguid.html. The EPA and the FDA are asking for public comment. (SB)

I would like to acknowledge Dr. Mike Ellis, Ohio State University, for some of the images used in this and other issues of Fruit Pest News.

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, Associate Professor and Extension Entomologist
fahale@ext1.ag.utk.edu

Both authors available at:
615-832-6802
fax 615-781-2568
Plant and Pest Diagnostic Center
5201 Marchant Drive
Nashville, TN 37211