Reports on Plant Diseases

RPD No. 942 - Gray-Mold Rot or Botrytis Blight of Vegetables

May 2000

Gray-mold rot or Botrytis blight, caused by the wide-spread fungus Botrytis cinerea, affects most vegetable and fruit crops, as well as a large number of shrubs, trees, flowers, and weeds.

The disease is favored by cool moist conditions and little or no wind. Cool, damp, poorly ventilated greenhouses are ideal for the disease, and Botrytis blights are probably the most common diseases of greenhouse-grown crops, especially in the spring and fall when the vents of greenhouses are closed at night to prevent heat loss. In the greenhouse, gray mold is often said to be a disease of bad management. Gray-mold losses in the field are severe following prolonged periods of overcast skies, fogs, heavy dews, or light drizzly rains.

The fungus causes primarily blossom blights and fruit rots, but can also cause damping-off, bud rot, stem cankers or rots, leaf spots or blights, bulb rots, and tuber or root rots. Botrytis is also a problem on fruits and vegetables in cold storage and subsequent shipment because the fungus is able to function at temperatures just above freezing. With some possible exceptions, Botrytis mainly attacks tender tissues (flower petals, buds, or seedlings), weakened or injured tissues, and aging (senescent) and dead tissues. Actively growing tissues, other than flower petals, are seldom invaded directly.

Symptoms

Symptoms of Botrytis diseases vary greatly depending on the host and plant part attacked. Generalized symptoms include a gray to brown discoloration, water soaking, and a fuzzy whitish gray to tan mold (mycelium and spores) growing on the surface of affected areas.

Blossom Blights and Fruit Rots

Blossom blights often precede and lead to fruit and stem rots. Aging flower petals of beans, carrot, celery, eggplant, onion, pepper, squash, and tomato are particularly susceptible to colonization by Botrytis species, and under cool, humid conditions abundant mycelium and conidia (spores) are produced on colonized petals. The fungus often grows from the fading flower petals into the rest of the inflorescence, or into developing fruit where it causes a blossom-end rot. From there it can spread and destroy part or all of the fruit.

Fruit can also be infected by conidia entering through growth cracks, cuts, stem scars, insect wounds, or lesions made by other pathogens. Infected fruit develop water-soaked, yellowish green or grayish brown irregular lesions which can be somewhat soft and spongy in texture. When conditions are favorable, mycelium and conidia are produced on the lesion surface. The fungus can infect the fruit of cucumber; squash; eggplant; pepper (Figure 1); snap (Figure 2), kidney, and Lima beans; lentils; and tomato (Figure 3) to name a few.

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Figure 1. Botrytis fruit rot of pepper. Note the typical dense gray mold on the decayed area (courtesy Dr. A.O. Paulus).

Figure 2. Gray mold lesions on pods of snap beans.

Figure 3. Young Botrytis lesions ("ghost spot") on green tomato fruits (courtesy BASF).

Figure 4. Botrytis blight has killed young lettuce plant.

Stem and Bud Rots

Stems or stalks of tomato and many other plants can be infected in the greenhouse or field through leaf scars or wounds, or leaf infections can progress into stem tissue. Stem lesions are dark, sunken, and elongated with distinct margins, or they may spread throughout the stem. In cloudy, wet weather infected tissues become covered with the typical grayish brown mold (Figure 5), and hard, black sclerotia may be produced. Asparagus "spears" are sometimes blighted and covered with the typical gray mold. Small, soft lesions can develop on the scale tips and actively growing buds of artichoke. Infections often progress from the buds down the stalk and into the lateral branches and main stem. Globe artichoke buds may be covered with a fine white mold, and when conditions are dry, a firm brown-to-black decay develops.

Bulb and Root Rots

Lesions can develop on any part of the root or bulb surface, but they are more likely to form at the crown, at wounds, or at the lower tip. Lesions usually appear soft, watery, and tan in color, later becoming somewhat spongy, dark brown, and lightweight. Eventually affected tissues may dry to form a grayish, leathery decay. Pockets of mycelium may develop between decayed bulb scales or on the surface of root lesions. Black sclerotia can also form on and in decayed tissue, except at low temperatures, when only a fine white mold develops. Gray mold rot causes considerable damage on stored carrot (Figure 6), parsnip, mangel, beet, endive, chicory, turnip, and rutabaga, but usually affects only topped roots. The bulb rot phase is common on onion (Figure 7) and garlic, but also occurs on other bulb crops.

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Outdoors Botrytis overwinters in the soil as mycelium on plant debris, and as black, hard, flat or irregular sclerotia in the soil and plant debris, or mixed with seed. The fungus is spread by anything that moves soil or plant debris, or transports sclerotia. The fungus requires free moisture (wet surfaces) for germination, and cool 60 to 77 F (15 to 25 C), damp weather with little wind for optimal infection, growth, sporulation, and spore release. Botrytis is also active at low temperatures, and can cause problems on vegetables stored for weeks or months at temperatures ranging from 32 to 50 (0 to 10 C). Infection rarely occurs at temperatures above 77 F (25 C). Once infection occurs, the fungus grows over a range of 32 to 96 F (0 to 35 C).

Masses of microscopic conidia (asexual spores) are produced on the surface of colonized tissues in tiny grape-like clusters (Figure 8). They are carried by humid air currents, splashing water, tools, and clothing, to healthy plants where they initiate new infections. Conidia usually do not penetrate living tissue directly, but rather infect through wounds, or by first colonizing dead tissues (old flower petals, dying foliage, etc.) then growing into the living parts of the plant.

Sclerotia are the primary means of survival in the field, and are viable within a temperature range of 39 to 131 F (4 to 54 C). The sclerotia generally produce conidia, and occasionally infection hyphae which can penetrate directly. The sclerotia can also produce apothecia (a sexual fruiting body), which in turn, produce ascospores that are also infective. Production of conidia and surface mycelium is inhibited by air movement at moderate humidity levels.

Control

1. Buy high-quality seed of recommended varieties. Treat the seed before planting.

2. Plant in a light, well-drained, well-prepared, fertile seedbed at the time recommended for your area. If feasible, sterilize the seedbed soil before planting, preferably with heat. Steam all soil used for plantbeds at 180 F (81 C) for 30 minutes or 160 F (71 C) for one hour.

3. Avoid heavy soils, heavy seeding, overcrowding, poor air circulation, planting too deep, over-fertilizing (especially with nitrogen), and wet mulches.

4. Strive for steady vigorous plant growth, not a soft luxuriant growth. Fertilize plants on the basis of a soil test.

5. Keep the greenhouse and seedbed soil somewhat dry after planting. Allow plants to get plenty of light. Keep the greenhouse, hotbed or cold frame glass clean. The glass sash should not leak water on the plants.

6. In greenhouses and seedbeds, provide for maximum air circulation, avoid excessive humidity, and do not allow water to form on the foliage. At night, maintain the greenhouse temperature higher than that outdoors, to prevent condensation of water on leaves. Some night heating may be necessary in Illinois through mid-June and starting again in early September.

7. Circulate the air with fans when heating greenhouses to keep warm, dry air in motion, and to prevent the development of stagnant air pockets. Botrytis will not be a problem if the humidity is kept below 90 percent.

8. Where possible, practice surface watering. Keep water off the foliage. Water plants early in the day to allow time for the foliage to dry before sunset.

9. Apply recommended fungicides when conditions favor disease development. Fungicides should be used with caution because the gray mold pathogen is often quick to develop fungicide-tolerant races; fungicides then only serve to suppress natural competitors, often making the disease more severe.

10. Where practical in greenhouses and gardens, carefully remove the first fruit infected with gray-mold rot from the growing area and burn or bury. Ideally, all diseased plants and plant parts should be removed and destroyed.

11. Avoid wounding or damaging the fruit during harvest and handling, and remove debris from storage areas. Store only blemish-free, clean vegetables in a cool environment where the humidity is high enough to prevent shriveling, but does not allow free moisture to form. The temperature for most vegetables should be as close to freezing as possible.

12. After harvest, where feasible, carefully collect, remove and burn, or cleanly plow under all crop debris.