black rot
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Beginner
Scientific Name
Guignardia bidwellii
Identification
Leaves
- small (2 to 10mm in diameter) lesions
- light brown lesions surrounded by a darker area with a marginal ring of small black pycnidia
- initially light brown then purplish brown to black
- become covered with black pycnidia
- dry up to become mummified
- most of the time will stay on the rachis
- under high disease pressure brown to black elongated lesions develop
- pycnidia may appear
Often Confused With
Berry abortion – shrivelled berries but no black pycnidia
Botrytis bunch rot – pinkish-brown berries covered by greyish-tan spores, no black pycnidia, skin seperates easily from flesh
Phomopsis – no berry infections apparent until veraison; foliar lesions much smaller, yellow with a dark brown centre and no pycnidia; shoot infections associated with longitudinal “razor blade” slashes
Gramoxone injury – tan to bronze coloured spots but with no pycnidia; most frequently occurring on foliage close to the ground during a herbicide application (dead weeds below the vines)
Period of Activity
Leaves are susceptible as soon as they emerge until until they are fully expanded. New foliar lesions can develop as long as new leaves are being produced. Berries are highly susceptible to infection for the first several weeks after they have formed. Concord berries become highly resistant to infection 4-5 weeks after bloom while
V. vinifera berries are slightly susceptible until 6-7 weeks after bloom, becoming resistant thereafter.
Scouting Notes
Scouting for foliar lesions should start when there are 4-5 fully expanded leaves. The first berry infections usually become apparent no earlier than pea-sized berry. Fruit infections can continue to be detected as late as veraison. Look for black pycnidia on the leaf lesions and berries as a distingushing feature.
Threshold
There is no threshold for initiating black rot control. Fungicides should be applied before infection occurs.
Advanced
Scientific Name
Guignardia bidwellii
Identification
Leaves
- small (2 to 10mm in diameter) lesions
- light brown lesions surrounded by a darker area with a marginal ring of small black pycnidia
- initially light brown then purplish brown to black
- become covered with black pycnidia
- dry up to become mummified
- most of the time will stay on the rachis
- under high disease pressure brown to black elongated lesions develop
- pycnidia may appear
Often Confused With
Berry abortion – shrivelled berries but no black pycnidia
Botrytis bunch rot – pinkish-brown berries covered by greyish-tan spores, no black pycnidia, skin seperates easily from flesh
Phomopsis – no berry infections apparent until veraison; foliar lesions much smaller, yellow with a dark brown centre and no pycnidia; shoot infections associated with longitudinal “razor blade” slashes
Gramoxone injury – tan to bronze coloured spots but with no pycnidia; most frequently occurring on foliage close to the ground during a herbicide application (dead weeds below the vines)
Biology
The fungus overwinters in canes and mummified infected berries. Ascospores are produced in pseudothecia in berries on the ground and in the trellis. Conidia are produced in pycnidia that develop in infected berries in the trellis as well as in cane lesions. When it rains in the spring, airborne ascospores are ejected from the pseudothecia and conidia are rain-splashed from pycnidia to infect new growth. Significant discharge of ascospores from mummies on the ground begins about 2 to 3 weeks after bud break and is usually complete within 1 to 2 weeks after the start of bloom. Mummies remaining in the trellis can continue to release conidia from the early pre-bloom period through veraison. Leaves are susceptible to infection until they are fully expanded. Lesions appear about 2-3 weeks after infection on leaves and pycnidia form within them 2-3 days later. V. vinifera berries are susceptible to infection from bloom through 6-7 weeks post-bloom and while V. labrusca are susceptible until 4-5 weeks post-bloom. Symptoms on fruit that are infected up to 4 weeks post-bloom develop within 14 days; however, if infection occurs late in the susceptibility period (about 5-6 weeks post-bloom), symptoms may not develop until as late as veraison. Berries that are infected very early in development will produce spores while healthy berries in the cluster are still susceptible to infection. These spores can wash to neighboring berries and cause epidemic spread of the disease under favorable weather conditions. The duration of wetness required for infection depends on the temperature with the shortest wetness period (6 hrs) at 26.5°C.
Period of Activity
Leaves are susceptible as soon as they emerge until until they are fully expanded. New foliar lesions can develop as long as new leaves are being produced. Berries are highly susceptible to infection for the first several weeks after they have formed. Concord berries become highly resistant to infection 4-5 weeks after bloom while
V. vinifera berries are slightly susceptible until 6-7 weeks after bloom, becoming resistant thereafter.
Scouting Notes
Scouting for foliar lesions should start when there are 4-5 fully expanded leaves. The first berry infections usually become apparent no earlier than pea-sized berry. Fruit infections can continue to be detected as late as veraison. Look for black pycnidia on the leaf lesions and berries as a distingushing feature.
Threshold
There is no threshold for initiating black rot control. Fungicides should be applied before infection occurs.
Management Notes
Selection of a site with good air drainage,north-south orientation of the rows to maximize sun exposure, and practices that open the canopy so that air circulation and spray coverage are improved can help control of black rot.
Pruning, training and leaf-removal to reduce shading and allow rapid air movement can reduce the frequency and duration of periods of high humidity which favour the disease. Canopy management can also enhance penetration of fungicide sprays.
Good weed control and proper soil drainage will reduce relative humidity and the amount of free moisture which may increase the spread of the fungus.
Removal of infected clusters from the vineyard as black rot symptoms develop during the growing season can reduce disease spread. This is a labour-intensive practice but a possible option for organic vineyards since none of the organically acceptable sprays are very effective against black rot.
Infected clusters and shoots should be removed from the canopy as soon as possible after harvest and either dropped to the ground and buried or removed and burned outside the vineyard. Do not keep them in a pile beside the vineyard without burning as spores can blow into the vineyard in the spring. Retain only healthy, lesion-free canes.
Hilling and dehilling as well as cultivation beneath the vines near bud break will bury mummies which will greatly reduce the number of spores that are released from them.
Management with fungicides – Fungicides are used to control black rot in most commercial vineyards. See OMAFRA Publication 360, Guide to Fruit Production - Chapter 5 Grapes (PDF): Recommendations for black rot at Trace bloom, Immediate post-bloom to early fruit set and Berries pea-sized. In vineyard that suffered severe infections the previous year, sprays should start at 3-5 leaves unfolded.
Different cultivars all have different susceptibilities to diseases. See Table 5-5 Relative Susceptibility of Grape Cultivars to Diseases.
Fungicides have different modes of action, and differ in their activity on grape diseases. See Table 5-6 Activity of fungicides on grape diseases.
