Fruit Pest News
Volume 3, No. 4 April 1, 2002
A weekly, 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. Current Conditions
2. Grapes: Pierce's Disease
3. Apples: Early-Season Fungicide Sprays
4. All Fruit: Be On Alert for Phytophthora Root Rot
1. Current Conditions
The Easter weekend was another wet one, and that's no April Fool's! The heavy rains we have experienced lately (three major periods since late January), may cause Phytophthora problems in fruit crops. We escaped major frost damage last week, with scattered frosts occurring on March 27 and 28. Cedar galls are now showing the bright orange tentacles that produce the spores of cedar-apple rust. If you have rust-susceptible apple varieties, you should spray immediately, if you have not already done so. This spray will also prevent primary scab infection.
Plant development is somewhat slow. Delicious and Golden Delicious at the Plant and Pest Diagnostic Center are at tight cluster. Redhaven peaches are in full bloom. Only two of ten blueberry varieties are in bloom. Matted-row strawberries are just beginning to bloom. (SB)
2. Pierce's Disease of Grape
Pierce's Disease (PD), a potentially destructive disease of grapes, was confirmed for the first time in Tennessee last year. Symptoms of PD had been encountered several times, but it had never been positively identified prior to last year's confirmation on a sample from a diseased vineyard in southeastern Tennessee. A hot-climate disease, PD is creeping northward as our winters continue to be milder than normal. PD is the most formidable obstacle to growing Vinifera grapes in the southern states. Other types of grapes are affected to a lesser degree.
PD is caused by the bacterium Xylella fastidiosa and is spread by certain kinds of leafhoppers. One particularly effective vector, the glassy-winged sharpshooter, has appeared in California and is causing a wine industry-threatening outbreak of PD in that state. X. fastidiosa can infect other types of plants. Many strains have been discovered. Some of these cause diseases of oak, sycamore, and maple (bacterial leaf scorch); plum (scald); peach (phony peach); and alfalfa (alfalfa dwarf).
The severity of PD appears to depend on climate. The bacterium appears to be sensitive to cold winter temperatures, and produces milder symptoms at higher altitudes, farther inland from ocean influences, and at more northern latitudes, even where vectors are plentiful. Please note that the vineyard in southeastern Tennessee had significant damage from PD, and many of the infected vines died.
PD symptoms begin as a drying or "scorching" of leaves http://csatwork.ucdavis.edu/Archives/Graphics/21600 srgb.jpg. The leaves become slightly yellowed along the margins before drying, or the outer leaf may dry suddenly while still green. Typically, the leaf dries progressively over a period of days to weeks, leaving a series of concentric zones of discolored and dead tissue. About mid-growing season, when foliar scorching begins, some or all of the fruit clusters may wilt and dry up. Leaves drop, but petioles (leaf stems) remain attached to canes for an extended period of time. The bark on infected canes often matures in irregular patches, resulting in "islands" of green surrounded by brown, or the reverse.
Control of PD consists of vector control and roguing (removing) infected plants. More information is needed on where PD now occurs in Tennessee, and where it is likely to occur in the future (and to what degree). Hopefully, we will be able to conduct surveys for the disease and for potential vectors in Tennessee. Your assistance in conducting these surveys will be greatly appreciated. (SB)
3. Early-Season Apple Fungicide Sprays
Many trees are now at tight cluster. For many orchards, this is the time for the first fungicide application. Scab control at this time is a must. If scab was abundant last year, the first application should be made at green tip. Scab should be treated early to prevent primary scab infections and thus break the disease cycle. Also, it is important to avoid early infections on sepals (calyx), as these are difficult to detect and can provide secondary inoculum throughout the early part of the growing season.
There are several different strategies for scheduling scab sprays. The selection of a spray program (what fungicide to use and how often) depends on factors such as other disease organisms present and the ability to obtain good coverage of the trees with the sprays. If cedar-apple rust or powdery mildew are a threat, it is probably best to control scab with a sterol-inhibiting fungicide (Nova, Procure, or Rubigan), because control of all three diseases would be obtained. Tank mixing with mancozeb or captan would improve the protectant ability and discourage the development of resistance to the sterol inhibitor (SI). The 3 to 4 day curative activity of the SI fungicides makes them useful at tight cluster in case some scab infections occurred prior to that time. The SI/protectant combination allows you to space the applications 7 to 10 days apart.
Where cedar-apple rust is a problem, mancozeb or ziram should be used as the protectant. The strobilurin fungicides Flint and Sovran can be used in lieu of the SI's, but I recommend reserving these materials for use during the summer. They may only be applied 4 times per year, and they play a more valuable role during the summer than during the spring.
Overwintering of powdery mildew is highly dependent on winter weather. Since we did not have prolonged cold temperatures this winter, there should be abundant inoculum in orchards that had mildew last year. If weather conditions are favorable (cool, wet) for mildew, disease pressure could be high on susceptible varieties. Be sure to use an SI on such blocks (see previous paragraph regarding the use of strobilurins). (SB)
4. Be On Alert for Phytophthora Root Rot
We have had several cases of excessive rain and saturated soil lately. Ponding of water has been common in poorly-drained and even in some not-so-poorly-drained soils. Such conditions are very favorable for spread of the Phytophthora fungus, which produces a swimming spore and can move from plant to plant or root to root. In plantings that have had Phytophthora root rot problems in the past, or even if susceptible varieties are grown, a treatment with Ridomil Gold may be advisable. Apples, peaches, blueberries, raspberries, and strawberries can have Phytophthora diseases. Refer to the Tennessee Small Fruit Spray Schedules for information on the formulation of Ridomil Gold and its rate for each crop. Ridomil should not be expected to revitalize trees showing moderate to severe Phytophthora root rot symptoms. Another application may be needed in the fall, after harvest. Also, Ridomil will not control root rots caused by fungi other than Phytophthora. (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, Associate 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
