Fruit Pest News
Volume 7, No. 15 June 20, 2006
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. Apple Fire Blight: Pruning Strategies in Young Apple Trees
2. Cucurbit Crops: Spray Program for Disease Control
3. The New Cucurbit Downy Mildew\
4. Cherry Leaf Spot
5. Raspberry Leaf Spot
6. Japanese Beetles
1. Fire Blight: Pruning Strategies in Young Apple Trees
Whether to prune out fire blight strikes during the summer or wait until the trees are dormant is a question with a lot of “it depends” attached to it. If the trees are young, however, most would agree: “Do it now.” There are still some caveats, however.
Fire blight infections can advance all the way to the roots and cause rootstock blight in young trees, resulting in tree death. Therefore, it is important to remove blighted shoots before the bacteria move systemically toward the roots. The importance of in-season pruning increases with varietal susceptibility, rootstock susceptibility, and tree age (the younger the tree, the more important the pruning).
Trees 5 years of age or younger should be watched closely. Go through the block and remove strikes 2 to 3 times a week until you don’t see any more strikes. Young trees may continue growing, and thus remain susceptible, until July or August, which is much longer than older trees.
It is important that growing season pruning operations be conducted in dry weather, as moisture can move the bacteria onto fresh cuts and assist their movement into the wood. Waiting for dry conditions mean holding off until heavy dew dries. It also includes holding off until rainy periods end, which can sometimes mean quite a delay.
Sterilizing pruning shears in alcohol or a bleach solution is considered optional. The inconvenience of sterilizing tools should not be allowed to interfere with the operation. If pruning during wet weather is necessary, sterilizing is encouraged. (SB)
2. Cucurbit Crops: Spray Program for Disease Control
Your strategy for spraying your cucurbit crop (watermelons, pumpkins, etc.) for disease prevention depends on the stage of growth of the crop. Elaborate spray programs are seldom needed on young crops. Following is the spray program I recommend:
Early season:
Use a primary fungicide (chlorothalonil, mancozeb, or maneb) every 7 to 14 days (more frequently in wet weather, less frequently in dry weather). Begin the program at vine tip-over to early bloom (3 to 4 weeks after seeding). Inspect the field for disease symptoms beginning at seedling emergence so that the spray program can be started sooner than planned, if needed. Also, look for insects regularly. If microdochium blight appears in pumpkin or squash, Flint or Cabrio should be alternated with the primary fungicide. Otherwise, there is little need to rotate the primary fungicides with other fungicides in the early season. (Chlorothalonil, mancozeb, and maneb are not subject to the development of resistance, so continued use is not a problem).
Mid-Late Season:
Scout for powdery mildew when this disease becomes a threat, around mid-summer. When the first powdery mildew colonies (circular, white patches) are seen, add sulfur, Nova, or Procure to the tank with the primary fungicide (preferably chlorothalonil, when powdery mildew is present). It is no longer recommended that the strobilurin fungicides (Flint, Quadris, Cabrio, Pristine) be used for powdery mildew control, because of resistance problems. Various diseases can occur in mid-late season, and the choice of fungicides should be determined by what diseases appear in the current year, or have occurred in the field in previous years. For example, Acrobat should be included if Phytophthora blight is in the picture. Add a fixed copper to the tank mix if angular leaf spot or bacterial leaf spot appear. The relative effectiveness of the labeled fungicides can be found in UT's Commercial Vegetable Disease, Insect and Weed Control publication. Air blast sprayers are needed when canopies become thick. Apply sprays every 7 to 14 days, depending on rainfall. (SB)
3. The New Cucurbit Downy Mildew
The experiences of the past two years have told us that the cucurbit downy mildew that we have been getting is not your father's downy mildew. It is more aggressive and difficult to control. Even resistant cucumber varieties are not as resistant as they used to be. It is more important than ever to keep a protective cover of chlorothalonil or mancozeb on your pumpkins or other cucurbit crops. Don't seed too thickly or apply too much nitrogen fertilizer. Some of the newer fungicides such as Tanos and Previcur Flex appear to be helpful, but none of the fungicides currently on the market are highly effective against the new downy mildew. (SB)
4. Cherry Leaf Spot
Cherry leaf spot is a problem in wet weather wherever cherries are grown. Tart cherries are particularly susceptible. The disease first appears as small, purplish spots on the upper surface of the leaves. These spots eventually turn brown. Most spots are circular; however, when abundant, they often coalesce and form large, irregular dead patches. The spots later separate from healthy tissue and drop out, leaving a "shot-holed" appearance. Frequently, there are areas around the spots that remain green, giving the leaf a mottled appearance. Leaves turn yellow and fall off.
Fungicides used for cherry leaf spot are captan plus Topsin M in the early cover sprays. Sterol inhibitors used in the preharvest period for brown rot control are also effective against leaf spot. If there is wet weather after harvest, dodine or chlorothalonil may have to be used once or twice, beginning 2 to 3 weeks after harvest. (SB)
5. Raspberry: Septoria Leaf Spot
Leaf spot, caused by Sphaerulina rubi, is often referred to as Septoria leaf spot (the asexual stage of the fungus is a Septoria species). It is a highly destructive disease that affects only raspberries. It was responsible for the failure of the highly susceptible variety 'Bababerry' in the Southeast. One of our main varieties, Heritage, seems to be fairly resistant.
Symptoms include small, tan to brown lesions, beginning on the lowest leaves and spreading up the plant. On more mature leaves, the lesions may become gray or silver. Heavily infected leaves turn yellow, become necrotic, and fall from the plant. Defoliation can be severe. The fungus overwinters primarily on dead leaves.
Coworkers and I conducted trials in two growing seasons in the early 1990's, in a Purple Royalty planting at the Middle Tennessee Experiment Station. The results of these trials provided information for making control recommendations, and provided data that supported the registration of Nova fungicide for control of Septoria leaf spot.
The only fungicide in the trials that was labeled at the time and provided
control was fixed copper (basic
copper sulfate was used in these trials). As a result, we have recommended copper
for control of this disease since 1993. Better control of Septoria leaf spot
has been made possible with the registration of Nova in 2000 and Captan 80WDG
in 2003. The Captan label does not include Septoria leaf spot, but good control
is possible. Captan 80WDG is labeled at 2.5 lb per acre, applied at 10- to 14-day
intervals, with a 3-day PHI and 72 hr REI.
Nova provides even better
control, but should be alternated occasionally with captan, Cabrio, Pristine, or copper to avoid
exceeding the 10 oz/acre/year limit
on the Nova label.
Use 1.25 to 2.5 oz/acre, depending on the amount of foliage present, and repeat every 10 to 14 days. Nova may be applied up to the day of harvest.
For best results, begin sprays before infections begin, which may be as early as April, and continue until fall. (SB)
6. Japanese Beetle Adults
Japanese beetles feed on many types of fruit, vegetable and ornamental plants as adults. They chew tiny holes in the leaves, often between the leaf veins. The leaves take on a lace-like or skeletonized look and significant foliar damage can occur over a short time. In grapes, blueberries, brambles, and strawberries carbaryl (Sevin) sprays need to be applied weekly to reduce the feeding damage. Repeat applications can be made up to a total of five times. In grapes, do not concentrate spray on the bunch or visible residues may result. Japanese beetles will also feed on corn silk and prevent proper pollination. Regular scheduled sprays for corn earworm will prevent most of this damage. Cover sprays for fruit trees will also limit the amount of leaf skeletonizing in most orchards. (FH)
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 2006 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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