FRUITAND
VEGETABLES
FRUIT
The information and recommendations in this newsletter are applicable to
North Carolina
and may not apply in other areas.
In two of six strawberry plantings visited last week in Lee and Harnett
Counties, two-spotted spider mites were present in numbers that indicated
the need for a miticide application. Cold injury was abundant and resembled
cyclamen mite injury. Some feeding by aphids and caterpillars were observed
in two fields. Harvest is about two weeks away, so fields should be checked
now and treatments made soon in order to meet time intervals between the
last spray and harvest. With the cold injury and a light crop, growers
are urged to offer plant protection to maximize profits.
Bacterial leaf spot, caused by Xanthomonas fragariae, is present in many fields this year. The need for overhead irrigation for frost-protection and excess water on strawberry leaves has been conducive to the spread and buildup of this disease. The disease can sneak up on growers because it is difficult to see early symptoms from the top of plants. Reflected light gives the appearance of healthy leaf tissues. However, to diagnose this disease, collect leaves and hold them up to the sunlight and view the leaf from the underside. Angular leaf spot causes translucent lesions between the veins and these lesions have an angular appearance. If a microscope is available, select tissue that has a lesion and slice it in the presence of water on a microscope slide using a razor blade. Cover the tissue with a cover slip and look under the microscope for bacterial streaming. You will see a cloud of bacterial ooze from infected sites. As the disease progresses, leaves turn brown and have a sticky feel due to the bacterial exudate.
Successful control of the disease is primarily dependent on the weather and cultural practices. If the weather warms up and there is limited rainfall, the disease will not spread extensively. However, if there is considerable rainfall, heavy dews or the need for additional overhead irrigation, the bacterial populations will continue to buildup. As the populations increase, the calyx of the fruit may become infected and turn black. This detracts from the aesthetic value of the crop. There is little evidence that the bacteria reduces yield or infects fruit. Depending on the weather, copper sprays (e.g., Kocide) can limit the buildup of the bacteria. If the weather is dry or excessively wet, copper may not provide any benefit. If copper is applied, growers need to be aware of label instructions concerning reentry.
If you diagnose bacterial leaf spot, please send specimens to the
North Carolina State University Plant Disease and Insect Clinic.
We hope to increase our collection of this pathogen for research purposes.
Research in Northeastern production regions (Ontario, Ohio, New York) has shown that one of the main sources of Botrytis spores is from dying or dead strawberry leaves. The spores become air borne and land on flower parts during bloom. Subsequently, the fungus invades the flower parts and spreads, especially from the stamens, into contiguous receptacles to initiate fruit rot. Direct infection of the fruit appears to be of limited importance . Early bloom sprays have proven highly effective for limiting the amount of sporulation and subsequent infection of the flowers and fruit. For example, in the Northeast two well-timed sprays at 10% bloom and again after 10 days appeared to be as effective as a scheduled spray program through the fruiting season.
The details of Botrytis infection cycles have not been extensively
documented for North Carolina plasticulture systems and our plants (mainly
Chandler) have a more extended bloom period than in the Northeast. However,
growers may want to consider two facts as they develop a gray mold control
strategy. First, disease pressure this year may be higher than other years
due to increased winter kill of strawberry leaves. Dead leaf samples, that
we collected from the ground or from plants this spring, readily and heavily
sporulated with Botrytis when brought into the lab. Such higher
inoculum levels could result in a high incidence of fruit mold, depending
on the weather conditions that will prevail during bloom. Second, the
fungus becomes established into the fruit primarily through flower parts
and early bloom sprays are an important consideration. Fungicide materials
and registered uses are listed in the North Carolina Agricultural
Chemicals Manual and specific fungicide labels.
Fruit blotch of watermelon is caused by the bacterial pathogen Acidovorax avenae subsp. citrulli and has caused widespread losses in regions where watermelons are grown. The pathogen is introduced to seedlings or plants from contaminated seed, volunteer plants and related weeds, or possibly from crop residue of the previous season. In the greenhouse, the most common source of bacteria is infested seed.
The disease may be on the greenhouse transplants but not show any symptoms. If symptoms do occur they may appear as water soaking (translucent areas) on the cotyledons. Later, these regions turn brown. Often, plants not showing the disease but harboring the bacteria can be set out in the field and the bacteria continues to multiply on the leaves without showing symptoms. Then as the young fruit develops, the bacteria enters the fruit through the rind and, as the fruit sizes, cracking and rot sets in. Mature fruit are not susceptible because they form a protective waxy layer. Fruit blotch can cause losses from 5 to 95%.
Most seed is now tested so the risk of disease is low. Growers should
ask their seed suppliers if the seed was tested for blotch. A helpful
practice is to keep different seed lots separate in the greenhouse, in
case one lot is contaminated and others are not. The bacteria spreads
rapidly with overhead watering so splashing should be minimized whenever
possible and plants should not remain wet for long periods of time
(e.g., all night). Copper sprays applied in the field have proven
highly effective in limiting the disease. However, because the risk of
disease is low, initiating a regular copper spray program is not necessary
or cost effective in most cases.
Web page last updated on 22 April 1996 by Stephen J. Toth, Jr..
