Fruit Pest News
Volume 3, No. 18 July 15, 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. Grape: Botrytis Bunch Rot
2. Apple: Bitter Rot
3. Peach: Brown Rot
4. Peach: Rhizopus Rot
5. Vegetable Corner
1. Botrytis Bunch Rot of Grape
Botrytis
bunch rot is particularly troublesome on tight-clustered French hybrids such as Vignoles
and Seyval, and most vinifera varieties, especially Pinot Noir, Riesling, and Chardonnay.
Hopefully, you made an application of a Botrytis fungicide before bunch closing,
when berries within the cluster begin to touch. Another application is needed at
veraison (beginning of fruit ripening), and another application can be made prior
to harvest, if needed. Removal of leaves around clusters before bunch closing has
been shown to reduce losses to Botrytis, by improving the drying conditions in the
cluster.
Three fungicides are registered for control of Botrytis bunch rot:
Rovral, Vangard, and Elevate. Realize that these products are effective ONLY against
Botrytis. You need to apply other appropriate fungicides for control of black rot,
bitter rot, the mildews, etc., all of which will continue to be active throughout
the summer. It is also important to remember that these Botrytis fungicides are prone
to resistance development in the pathogen population. It is therefore strongly recommended
that none of these materials be used more than 3 times per year, despite the fact
that the Rovral label allows 4 applications (the Vangard label limits its use to
2 applications per year, at the 10 oz/A rate). Try to alternate the 3 products, if
possible.
Rovral 50WP is used at 1.5 to 2 lb/A, with a 7-day pre-harvest interval.
Vangard 75WG is used at 10 oz/A, with a 7-day PHI. Although Vangard is classified
as a reduced-risk pesticide, it is toxic to fish and should be used with caution
around aquatic areas (see label). Elevate 50WG is used at 1 lb/A, and has a 0-day
PHI. (SB)
2. Bitter Rot of Apple
Bitter
rot is the most potentially explosive of the apple fruit rot diseases. We sometimes
get reports of growers, especially homeowners, losing the majority of their fruit
to this disease. The restrictions placed on the most effective group of fungicides,
the EBDC's, have increased our problems with bitter rot. The disease can move so
rapidly because infection can occur in as little as 5 hours of wetting at 79-82 F.
At 80 F, lesions can develop and produce spores within 11 days of infection. Fruit
remain susceptible right up until harvest.
Identification. Bitter rot is recognized by round,
sunken lesions on the fruit. When very young fruit are infected, the lesions
appear as tiny gray-brown flecks, which usually do not develop further until fruit
begin to ripen. Fruit infections occurring a month after petal fall begin as small,
round, slightly sunken areas that are light to dark brown in color. As the lesions
enlarge, concentric rings of spores or their fruiting bodies form around the center.
Decay lesions extend in a cone-shaped pattern toward the core, observed by cutting
the apple in half through the decayed area. This V-shaped lesion distinguishes bitter
rot from white rot, which has a cylindrical decayed area.
Cultural
control. A feared disease,
bitter rot is controllable if trees are well-pruned and air circulation is good in
the orchard. The removal of excess growth improves drying conditions and penetration
of sprays into the canopy. Try to prevent the buildup of inoculum by removing overwintered mummified
fruit, if feasible, or even diseased fruit during the growing season. Leaving
fire-blighted shoots on the trees can increase fruit rot problems, because the
fungi can colonize them. Do not allow prunings to pile up near the orchard. Prunings
can be left on the orchard floor if they are chopped with a rotary or flail mower,
which removes the bark on which the fungus can reproduce.
Monitoring bitter rot. Orchards should be scouted regularly
for this and other fruit rots. In orchards in which this disease has been a problem,
or in highly susceptible varieties, inspect the trees each week, beginning in early
June. Highly susceptible varieties include Golden Delicious, Fuji, Arkansas Black,
Granny Smith, Empire, and Yellow Newton. Almost as susceptible are Gala, Stayman,
Jonagold, Jonathan, Grimes Golden, Ginger Gold, and Braeburn. Among the least susceptible
are Delicious, Rome Beauty, and Winesap, but no variety has a high level of resistance.
Inspect
as many fruit as is practical (this exercise provides another case for keeping trees
trained to a manageable height). If more than 3 infected fruit are observed on any
tree, a change in the control program may be needed. The needed change may be in
the tree training and other cultural practices mentioned above.
Or it may be in the spray program. The spray interval may need to be shortened (especially
if the weather is rainy), the fungicide choice may need to change (a bitter rot fungicide
should be used in each cover spray), or the application method may need adjustments.
Chemical
control. The need for fungicidal protection against bitter rot is greatest from
midseason until harvest, although some fruit infection can occur as early as petal
fall. Where bitter rot has been troublesome in the past, use an EBDC fungicide (Dithane,
Manzate, Penncozeb, or Polyram) as long as the label allows (until 77 days before
harvest). Tank mixing with captan after petal fall will improve control of white
rot and black rot.
Ferbam is also very effective against bitter rot, and can
be used until 7 days before harvest. However, ferbam produces a gray residue that
is unsightly on fruit. Other disadvantages of ferbam are increased cracking on Stayman,
and russeting on certain varieties if ferbam is applied in the pink through first
cover sprays. Russeting is not a problem when ferbam is used after the first cover
spray. Like the EBDC's, ferbam applied alone may not provide adequate control of
white rot or black rot.
Captan, ziram, and thiram are good bitter rot materials.
The new strobilurin fungicides, Flint and Sovran, appear to be slightly less
effective at the rates commonly used. Cover sprays should be applied
at 10- to 14-day intervals, using 10-day intervals during wet, rainy periods. (SB)
3. Brown Rot of Peach
Remember
that special attention to brown rot is needed in orchards in which bird or insect
damage is severe, and in which trees are planted closely or in which woods surround
the orchard. Post-harvest losses to brown rot can be significant, especially if harvested
fruit are not kept cool. Try to minimize post-harvest losses by using care in the
picking and handling of fruit to avoid punctures. Any break in the skin of the fruit
enables brown rot to more easily cause infection.
A good pre-harvest spray
program also minimizes losses at harvest and after harvest. There currently is a
dilemma in that our 3 most effective brown rot fungicides (Orbit, Indar, Elite) belong
to the same chemical class, the sterol inhibitors. As a result, resistance to these
fungicides may develop. Brown rot resistance has developed to the benzimidazoles
(Benlate, Topsin M), causing these formerly outstanding materials to be unreliable
in many southeastern peach orchards. We should make every effort to minimize the
use of the sterol inhibitors, preferably by alternating them with Abound, which belongs
to the strobilurin group. Pre-harvest fungicides should not be applied more often
than every 7 to 10 days. Orbit, Indar, Elite, and Abound have 0-day pre-harvest intervals.
(SB)
4. Rhizopus Rot of Peach
Rhizopus
rot is a common post-harvest rot of peach that can spread rapidly in containers of
non-cooled fruit. It is characterized by a fluffy, black fungal growth on a rotted
area of the fruit. Before the mold is produced, Rhizopus rot can be detected by an
easy slippage of the skin from the rotting flesh. Skin slippage does not readily
occur over brown rot lesions.
Brown rot fungicides used during the pre-harvest
period are not effective against Rhizopus rot. Use Botran 75W in a tank mix with
a brown rot fungicide, but not within 10 days of harvest. As with brown rot control,
avoid puncturing fruit skin, and store harvested fruit at less than 40 F. (SB)
5. Vegetable Corner
We are seeing lots of southern blight on tomatoes, peppers, and potatoes. This soil-borne fungus is favored by the hot weather we have been having. Southern blight is characterized by a rapid wilt of the entire plant, followed by death. In some cases, a strong, white, fan-shaped mold can be seen on the stem at the ground line, where the fungus attacks the plant. Avoid pulling soil (which may contain the fungus) up around the plant by cultivation. At the end of the growing season, turn the soil with a turning plow one time, then conduct no more cultivation. Do not use a rotary tiller. The southern blight fungus is sensitive to deep burying. Terraclor can be used at planting time.
Tomato spotted wilt virus is widespread on tomatoes in much of the state. Unfortunately, little can be done to control this thrips-transmitted virus. Leaf veins are purple on the undersides, and the leaves in the upper part of the plant may be blighted. Blighting often begins as bronze to purple spots on leaves. Green fruit may show dark lesions or a sooty appearance. Ripe fruit show yellow rings. (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