The diseases known as Botrytis Rot, Gray Mold, Bunch Rot, and Noble Rot on Grapes are all caused by the fungal pathogen, Botrytis cinerea. This fungus has wide range of host plants and is common throughout the world, wherever grapes are grown. On grapes it can grow on flowers or berry but can also infect leaves and shoots. In some cases, for specific dessert wines, Noble Rot, cause by Botrytis cinerea can be desirable, for its sugar production and unique flavor characteristics. In most cases though, Botrytis infections are seen as undesirable and disease epidemics can be very economically damaging.
Botrytis Grape Rot Symptoms
Botrytis most commonly affects younger, tender leaves, usually in a cooler, wet spring. Infected leaves first show patches of light brown or tan, softened tissues. These infected areas die, causing plant wilt.
Infected canes also develop softer brown patches at infected areas. As the disease develops, gray spores may form on the canes, and later in the season, dark black sclerotia may be present.
Flower clusters are also very susceptible to botrytis infection, especially following periods of rainfall. Infected flower clusters develop velvety gray spores before drying out and dying. Sometimes the disease will remain dormant until verasion. At verasion, infected berries will turn brown for white grapes or red for for red varietals. If environmental conditions are acceptable, infected berries will develop the characteristic gray, velvety spores. These spores can then spread and infect other berries. In severe instances, the grapes may collapse and appear to be raisin-like
Conditions for Botrytis Rot On Grapes
Botrytis cinerea can overwinter through sclerotia, dormant black structures that are able to survive long periods of time. In vineyards, sclerotia are most commonly found on canes infected in autumn, or in mummy berries that have not been removed from the field. In spring, these sclerotia can germinate and produce spores which are able to infect a variety of plant tissues. These spores are spread by wind, or equipment or personnel moving between fields.
In order to grow and reproduce spores, Botrytis cinerea requires periods of wetness with cooler, moderate temperatures. Free water can be from rain, dew, overhead irrigation, or juice from damaged grapes. For these reasons, botrytis infection is more common in coastal areas, or in areas with heavier rainfall.
UC Davis researchers have developed a model to help track risk of infection by using leaf wetness duration and temperature data. The Pest Prophet app can be used to easily obtain weather data for your vineyard and determine risk of botrytis infection. This information can be used to determine if preventative fungicide sprays are necessary, and when the optimum timing would be to spray.
Management of Botrytis Rot on Grapes
The first step in botrytis management for grapes will occur even before planting. Common grape varietals have different levels of susceptibility. Many red cultivars can produce an enzyme which can inhibit botrytis reproduction. Cluster architecture can also affect botrytis infection and rate of spread. (For example Zinfadel is more susceptible than cabernet sauvignon, and the primary reason is it’s tight clusters.) In coastal or wet rainy areas, where botrytis control may be more challenging, or in organic vineyards, highly susceptible varieties may want to be avoided. When planting different varietals in the same vineyard, specific micro-climates of different regions of the vineyard should be considered when choosing where to plant each varietal. Areas with higher humidity, less heat, and less wind movement should be planted with more resistant grape varieties.
Cultural Management Techniques
Row orientation, plant spacing, and trellising are very important to preventing widespread botrytis outbreaks. Wind movement should be maximized which will help plants to dry out faster, and reduce the available germination time for botrytis. Canopy management through pruning techniques, shoot -thinning, fertilization and irrigation management can also be very important in botrytis management. Large over-grown canopies will have drastically different -within the canopy which may lead to botrytis outbreaks. Canopy management can also play a large role in spray coverage and efficacy.
Sanitation efforts for botrytis can also play a large role in preventing infections. Removing infected canes, leaves, and fruit completely from the field will reduce the level of conidial spores within the season and sclerotia which can overwinter in the field. If removing infected plant parts form the field is not economically feasible, they should be extensively incorporated into the furrows, to reduce the amount of inoculating sources present in the field.
Avoiding physical injuries to berries and leaves is also important, because botrytis usually first infects wounded or damaged tissues. Mechanical injuries during pruning, shoot-thinning, weeding or other operations should be carefully avoided. Most injuries on the berries will occur from insects such as the omnivorous leaf-roller.
Chemical and Bio-control Options
Fungicides can also be used to control botrytis. There are many different types of fungicides with different modes of action, but most will be much more effective as preventative sprays, before infection has occurred. Lime sulfur is a good option during the winter season, because it disrupt the overwintering botrytis sclerotia. During the growing and fruiting seasons, preventative sprays should be used when weather conditions are optimal for infection and growth. In some areas of California and other wine growing regions, resistance has been observed against certain classes of fungicides, so rotating chemicals and active ingredients is very important for botrytis control.
There are also bio-control options using different types of beneficial bacteria that can help control the spread of botrytis rot on grapes. There are also some interesting new innovations for applying these bio-control agents. Products that boost grape’s natural immunity responses may also be helpful, though less is known about the efficacy or practicality of these practices.
Learn More: Complete Guide to Botrytis (Gray Mold)
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