Fruit Pest News
July 23, 2001
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. Plum Pox Survey
2. Blueberry: Stem Blight
3. Grape: Bitter Rot
4. Apple: Sooty Blotch and Flyspeck
1. PLUM POX SURVEY
Anni Self, of the Tennessee Department of Agriculture, reports that approximately 200 peach tree samples were tested for plum pox virus this year, and all were negative. The survey will resume in September. Testing efficiency for the virus drops as the temperatures rise, so the survey is suspended in the summer. Please give Anni your complete cooperation in this survey for this important stone fruit disease (SB).
2. STEM BLIGHT OF BLUEBERRY
Stem
blight, caused by the fungus Botryosphaeria dothidea, is one of the most important
disease of blueberries in Tennessee. It is especially troublesome in young plants,
but can occur on any age blueberry plant.
Stem blight is recognized by a
rapid wilt and death of individual branches, sometimes followed by death of the entire
plant as the fungus spreads downward internally to the base of the plant. Branches
recently killed do not drop their leaves, resulting in a brown-leafed "flag"
that stands out against the green healthy portions of the bush. By removing an affected
stem and splitting it longitudinally, you can see a light brown discoloration in
the wood, extending down the infected side of the stem. Stem blight does not produce
cankers on the stem surface. Click
here to see images of these symptoms produced by North Carolina State University.
Click on the images to enlarge them.
The fungus infects many types of woody
plants in addition to blueberry, and overwinters in infected stems. Spores are produced
throughout the entire growing season, and are spread by wind and rain. Infection
can occur at any time during the growing season, with the greatest number of infections
in early summer. The fungus enters stems through wounds, such as pruning cuts, borer
holes, and winter injury.
Control of stem blight depends on cultural methods.
Fungicidal sprays are not recommended for two reasons: None of the labeled fungicides
are very effective against this disease, and the frequent sprays that would be needed
for control would have to be maintained for the entire growing season.
3. BITTER ROT OF GRAPE
If
you are seeing a rot of mature berries (8% sugar or above), the culprit is probably
bitter rot. Black rot, which has a similar appearance, occurs in green berries. Bitter
rot can be a particular problem to wine grapes because of the unpleasant flavor it
can impart to the wine. Catawba is particularly susceptible.
The causal fungus
usually invades a berry from the pedicel (berry stem). As
the berry rots, concentric rings of fungal fruiting bodies appear. Within a couple
of days, the berry softens and is easily detached. Berries that do not fall off continue
to dry and become firmly attached. When shriveled, the berries look much like berries
affected by black rot or other fruit rots.
The bitter rot fungus overwinters
on fallen leaves and berries and in the bark of year-old canes. The disease cycle
on fruit starts shortly after flowering, when the fungus invades the natural warts
on the berry stems and remains latent until the berry reaches maturity. It then invades
the stem and moves into the berry, where a rot forms and spores are produced on the
surface. Spores that are splashed to injured fruit cause new infections.
Generally,
broad-spectrum fungicides such as Benlate, captan, ferbam, and mancozeb are effective
in controlling bitter rot. Used around bloom time and afterward, these materials
help prevent pedicel infections. Late season sprays are needed to prevent secondary
infections. (SB)
4. SOOTY BLOTCH AND FLYSPECK OF APPLE
Benlate, Topsin M, Flint, and Sovran should be included in all cover sprays after sooty blotch and flyspeck first appear. Recent in studies in New York indicate that Flint and Sovran have residual activity against sooty blotch and flyspeck similar to that of Benlate and Topsin M. Tank mixing with captan, ziram, or thiram is not necessary for Flint and Sovran, as it is for Benlate and Topsin M. Furthermore, Flint and Sovran fill certain niches such as providing control of Alternaria leaf blotch and secondary scab. However, they are not effective against black pox or necrotic leaf blotch. If used on Golden Delicious, it would be wise to include zinc oxide in the spray tank to suppress necrotic leaf blotch. Sovran has a lengthy (30-day) pre-harvest interval (PHI), and neither product can be used more than 4 times per year, or 3 times consecutively. Once harvest begins for a cultivar, only one of the above four fungicides (Topsin M) can be used, because of harvest restrictions. Most sooty blotch and flyspeck infections that take place after harvest begins will not have time to become visible. However, an application of Topsin M (0-day PHI) near harvest can eradicate infections that are in their incubation period. (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
