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Gray mould (botrytis)

Close-up showing the fungus sporulating on berries

Many fungi are capable of rotting mature or near-mature fruits of strawberry, raspberry, and blackberry. Under favorable environmental conditions for disease development, serious losses can occur. One of the most serious and common fruit rot diseases is gray mold. The gray mold fungus can affect petals, flower stalks (pedicels), fruit caps, and fruit. In wet, warm seasons, probably no other disease causes a greater loss of flowers and fruit. The disease is most severe during years with prolonged rainy and cloudy periods during bloom or during harvest.

Symptoms

Young blossoms are usually very susceptible to infection. One or several blossoms in a cluster may show blasting (browning and drying) that may extend down the pedicel. Fruit infections usually appear as soft, light brown, rapidly enlarging areas on the fruit. If infected fruits remain on the plant, the berry usually dries up, "mummifies," and becomes covered with a gray, dusty powder, which gives the disease its name "gray mold." Fruit infection is most severe in well-protected areas of the plant, where the humidity is high and air movement is poor. On strawberry, berries resting on soil or touching another decayed berry or a dead leaf in dense foliage are most commonly affected. The disease may develop on young green fruits, but fruits become more susceptible as they mature. Usually, the disease is not detected until fruits are mature at harvest time. After picking, mature fruits are extremely susceptible to gray mold, especially if bruised. During picking, the handling of infected fruit will spread the fungus to healthy ones. Under favorable conditions for disease development, healthy berries may become a rotted mass within 48 hours after picking.

Causal Organism

Gray mold is caused by the fungus Botrytis cinerea. The fungus is capable of infecting a great number of different plants. The disease cycle is very similar for both strawberries and brambles. The fungus overwinters as minute, black, fungus bodies (sclerotia) or as mycelium in plant debris, such as dead strawberry or raspberry leaves. Recent research has shown that nearly all of the overwintering inoculum in strawberry plantings comes from mycelium in dead strawberry leaves within the row or planting. In early spring, the mycelium becomes active and produces large numbers of microscopic spores (conidia) on the surface of old plant (leaf) debris in the row. Spores are spread by wind throughout the planting where they are deposited on blossoms and fruits. They germinate when a film of moisture is present and infection can occur within a few hours. Temperatures between 70 and 80 degrees F (20 to 27 degrees C) and free moisture on the foliage from rain, dew, fog, or irrigation water are ideal conditions for disease development. The disease can develop at lower temperatures if foliage remains wet for long periods. Strawberries and raspberries are susceptible to Botrytis during bloom and again as fruits ripen. Recent research indicates that most fruit infection actually occurs during bloom; however, symptoms usually do not develop until close to harvest. During bloom, the fungus colonizes healthy or senescing flower parts, often turning the blossoms brown. These blossom infections establish the fungus within the receptacle of the young fruit as a "latent" or "quiescent" infection. The fungus generally remains latent in developing (green) fruit until the fruit starts to mature, at which time the fungus becomes active and symptoms (rot) appear. Thus, the most critical period for applying fungicides to control gray mold is during bloom. This is an important point to remember when considering fungicide applications for controlling this disease.

Control

Select a planting site with good soil drainage and air circulation. Plants should be exposed to direct sunlight. Plant rows with the direction of the prevailing wind to promote faster drying of foliage and fruit.

A good layer of straw mulch (or other material) between the rows or around the plants aids greatly in controlling fruit rots. The mulch acts as a barrier that reduces fruit contact with the soil.

Proper spacing of plants and timing of fertilizer applications are also important. Excessive applications of nitrogen fertilizer, especially in the spring before harvest, can produce excessive amounts of dense foliage. Shading of berries by thick foliage prevents rapid drying of the fruit during wet periods and creates ideal conditions for disease development.

Good weed control is very important. Weeds prevent air movement in the plant canopy. This slows drying time of flowers and fruits and increases the chances for infection. Pick fruit frequently and early in the day as soon as plants are dry. Cull out all diseased berries but do not leave them in the field. Handle berries with care to avoid bruising. Refrigerate fruit promptly at 32 to 50 degrees F (0 to 10 degrees C) to check gray mold.

Fungicides are an important disease management tool in commercial plantings, but are generally not effective unless they are timed properly and used in conjunction with the above mentioned cultural practices.

Botrytis bunch rot is caused by the fungus Botrytis cinerea. This fungus is very common in nature, and causes diseases on a variety of unrelated crops. Bunch rot can cause serious losses on highly susceptible grape varieties. Although berries of all grape varieties are susceptible to bunch rot, losses generally are greater on tight-clustered varieties of Vitis vinifera and French Hybrids. Losses result from the rotting of berries in the field or in storage.

Symptoms

Infection of ripe berries is the most common and destructive phase of this disease. Infected berries first appear soft and watery. The berries of white cultivars become brown and shriveled, and those of purple cultivars develop a reddish color. Under high relative humidity and moisture, infected berries usually become covered with a gray growth of fungus mycelium. One or a few berries within the bunch or the entire bunch may be affected. Generally, healthy berries touching infected berries will become infected. Rotted berries generally shrivel with time and drop to the ground as hard mummies. The fungus also can cause a blossom blight that can result in significant crop loss early in the season. Although uncommon, leaf infections also occur, but appear to be of no economic importance in Ohio. Leaf infection begins as dull, green spots, commonly surrounded by a vein. The spots rapidly become necrotic lesions.

Casual Organism and Disease Cycle

Botrytis bunch rot is caused by the fungus Botrytis cinerea. The fungus overwinters in grape mummies, dead grape tissues, and other organic debris in and around the vineyard, as well as on a multitude of alternate plant hosts. Because of its wide host range, growers always should assume that the fungus is present in the vineyard. In spring, the fungus germinates from small, dark, hard resting structures known as sclerotia. The fungus then produces spores (conidia) that spread the disease. These spores are produced throughout the growing season. As blooms die, the spores germinate and colonize dead flower parts. Using the dead tissue as a food base, the fungus invades living tissue. After penetrating the berry, the fungus may remain dormant until the fruit sugar content increases and the acid content decreases to a level that supports fungus growth. Symptoms then develop readily under warm, moist conditions.

Berries that escape bloom-time infection may become infected at or near harvest under favorable environmental conditions. Any wound on the berry provides an excellent infection site for the fungus even in the absence of favorable environmental conditions. Birds, insects, hail, and powdery mildew are common causes of wounds. Swelling during ripening in tightly packed clusters causes pressure that also can rupture the berries. Wet and humid conditions around the berries and leaves greatly enhance disease development. The longer wet conditions persist, the greater the probability of infection, even to undamaged berries. Warmer temperatures also favor infection. At 54 to 75 degrees F, infection occurs in 12 to 24 hours, while at 37 degrees F, 60 to 72 hours are required.

Disease cycle of Botrytis bunch rot

Figure 3. Disease cycle of Botrytis bunch rot. Source:New York State Agricultural Experiment Station, Grape IPM Disease Identification Sheet No. 3.

Control

  1. Promote good air circulation and light penetration by proper pruning, controlling weeds and suckers, and positioning or removing shoots for uniform leaf development. Where possible, rows should be planted in the direction of the prevailing wind. Good air circulation and light penetration promote faster drying of plant parts and reduce the risk of disease. Removal of leaves around clusters on mid- or low-wire cordon-trained vines before bunch closing has been shown to reduce losses caused by Botrytis in New York and California vineyards, due to improved air circulation and improved spray penetration and coverage.
  2. Prevent wounding by controlling insects, birds, and other grape diseases.
  3. Growth regulators that lengthen the rachis and separate the berries in tight-clustered cultivars can reduce the damage from berries being crushed within the cluster; thus, reducing infection and spread of Botrytis.
  4. In commercial vineyards, effective fungicides applied at appropriate times during the growing season provide significant control.

Source: Ohio State University Extension Factsheet, Plant Pathology
Botrytis Bunch Rot or Gray Mold of Grape, HYG-3025-95
Michael A. Ellis

Without a doubt the most common disease of greenhouse floral crops is gray mold. The disease can affect almost every type or variety of floral crop grown. This disease can either be a common nuisance or an economic disaster depending on the host and the conditions under which the crop is grown. It is also one of the easiest diseases to control using nonchemical means.

Symptoms

Symptoms of gray mold vary depending on the host and the environmental conditions associated with the host. Under most conditions and with most hosts the disease is characterized by the production of leaf spots, flower blight, bud rot, stem canker, stem and crown rot, cutting rot, damping off, and in extreme cases, plant death. When conditions of high relative humidity prevail, at or above 85%, the fungus can be seen growing and sporulating on the infected tissue. Fungal growth is characterized by the presence of fluffy, gray/brown mycelium that produces a cloud of spores if disturbed. Affected tissue is soft and brown, and sometimes has a water-soaked appearance.

Botrytis flower blightBotrytis flower blight on Geranium

Flower Blights and Bud Rots

Flower blight of greenhouse crops such as begonia, carnation, chrysanthemum, cyclamen, geranium, impatiens, marigold, and petunia often precedes and leads to stem rot. The fungus becomes established in the flower petals and under ideal conditions moves from the flower to the pedicel or peduncle, and eventually to the stem. Symptoms appear as tannish, irregular spots on the flower petals. Flowers can also become infected in the bud stage. The buds turn brown and appear to be water-soaked. Infected buds fail to open and may sometimes abort.

Leaf Spot and/or Blight

Leaf spot often appears when infected flower petals or other plant parts fall on the leaves. The pathogen can also invade damaged tissue on leaves that have marginal or tip burn. When this happens, the affected leaf may develop a triangular-shape lesion. Other leaf spots appear water-soaked, are brown in color and have irregular margins.

Botrytis stem blight on VincaBotrytis stem blight on Vinca

Damping-Off or Bed Rot

Damping-off of floral crops can commonly be associated with gray mold. Susceptible hosts include, cineraria, cyclamen, exacum, and snapdragon. Damping-off is prevalent when the relative humidity is high or when the growing medium is contaminated with the fungus. Infected seedlings wilt or collapse at or near the soil line.

Causal Agent and Disease Cycle

Gray mold is caused by the fungus Botrytis cinerea. It is a common fungus, with a very wide host range and can persist in the greenhouse year-round as mycelium, conidia, or as sclerotia on living or dead tissue. The fungus produces a large amount of spores that move throughout the greenhouse via air currents. Under optimum environmental conditions (relative humidity at or above 85%, with little or no air circulation or with free water on the leaf surface), the spores land on the plant surface, germinate, and penetrate the host plant. The optimum temperature for spore germination is 72 to 77 degrees F (22 to 25 degrees C). Germinating spores rarely penetrate actively growing tissue directly. However, penetration of actively growing tissue can take place through wounds. Cutting stubs are particularly susceptible to gray mold infection. Symptoms, like those described earlier, will appear on infected plants within a few days. If left unchecked the fungus will grow and sporulate and the newly produced spores will be the source of infection for other hosts in the greenhouse. The fungus can also be a post-harvest problem, becoming established at temperatures of 32 to 50 degrees F (0 to 10 degrees C).

Disease Management

Control the Environment

If there is one practice that will go a long way toward the management of gray mold, it is controlling the environment. Maintaining an environment within the greenhouse that will not permit the fungus to grow and sporulate is essential to control. By keeping the relative humidity below 85%, as well as maintaining good air circulation and adequate plant spacing, excellent control can be achieved. Whenever possible, plants packed closely together should be spread apart to allow better air circulation and to reduce relative humidity within the plant canopy. Fans should be used to provide good air movement above the canopy. Plants with wounds should be either protected with a fungicide or removed from the greenhouse, as the wound is the perfect environment for the fungus to initiate the infection process.

Sanitation

Infected plant material should be removed from the greenhouse so that it is not a source of inoculum for the rest of the house. Infected plant material should not be allowed to sit in trash cans within the house as the fungus will continue to grow and sporulate on the dead and dying tissue. Subsequent opening and closing of the trash cans will produce enough air movement to release spores out into the greenhouse.

Figure 5. Botrytis blight on Poinsettia.Figure 6. Botrytis sporulating.

Botrytis Blight or Gray Mold

The plant pathogenic fungus Botrytis is found virtually everywhere plants are grown. It is fast growing, can grow on many different sources of nutrients, survives well in the greenhouse, and can attack many different types of plants. The disease caused by Botrytis is commonly called Botrytis blight or gray mold.

The Fungus

Botrytis at first appears as a white growth on the plant but very soon darkens to a gray color. Smoky-gray spores form and are spread by the wind or in water. In greenhouses, any activity will result in a release of spores. Even automated trickle irrigation systems, when turned on, trigger a release of spores. These spores are often found on the outside of seeds. The spores can remain dormant on plant surfaces as long as the life of the plant in some cases. Botrytis forms two types of resting structures on or in infected plant tissue: (1) very dark-brown or black multicelled structures called sclerotia and (2) single-celled, thick, dark-walled chlamydospores. The fungus can persist in the greenhouse for long periods in either structure in the absence of plants.

Infection

Botrytis is a weak pathogen that must have nutrients or some food source before it invades the plant. Nutrients leaking from wounded plant parts or from dying tissue such as old flower petals provide the required nutrients. From this food base, the fungus becomes more aggressive and invades healthy tissue. A dark- to light-brown rot forms in the diseased tissue. High humidity conditions favor the growth of this fungus.
Sites of First Infection

  • Wounded tissue such as large stubs left after taking cuttings.
  • Fading flowers.
  • Leaves on which fading infected flowers have fallen.
  • Broken stems or injured leaves.
  • >Leaves damaged by overfertilization, spray damage, or mechanical injury.
  • Seedlings grown under cool, moist conditions.
  • Cuttings taken from plants with heavy infestations of Botrytis.

Management

  • Sanitation is the first important step. Remove dead or dying tissue from the plants and from the soil surface. Remove this refuse from the greenhouse. Do not throw debris under benches or on walks. Sanitation alone is not sufficient to control this fungus. The fungus can produce 60,000 or more spores on a piece of plant tissue the size of your small finger nail. Even one spore can infect a plant and cause disease.
  • Avoid injuring plants in any way. Do not leave large stubs of tissue on stock plants when taking cuttings.
  • Heat and ventilate greenhouses to prevent high humidity conditions. This may only require extra venting early in the day when moisture has condensed and before sunlight has warmed the air. Even lowering the humidity slightly can have a significant effect on Botrytis. Outdoor planting should be planned to provide good air circulation patterns. This is the most important means of inhibiting Botrytis activity.
  • Added protection is available for many crops by applying a fungicide or combination of fungicides. However, Botrytis can develop resistance to certain chemicals.

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