Grape white rot

Grape white rot is a fungal disease of grapevines caused primarily by Coniella diplodiella. It most often damages ripening clusters after hail, insect injury, cracking, or prolonged wet weather. Infected berries develop expanding water-soaked lesions, soften, shrivel, and become covered with pale gray to whitish pycnidia. Accurate diagnosis can require laboratory confirmation because several Coniella species cause similar grape symptoms.

Grape white rot is most damaging to berries and bunches, especially from fruit set through harvest and commonly near ripening. The disease can also affect leaves, pedicels, rachises and green shoots. It is favored when warm, humid weather follows rainfall or hail, but hail is not required for disease development.

Because symptoms overlap with other grape rots, including infections caused by Botryosphaeriaceae, Phomopsis and other Coniella species, a definitive species-level identification should be based on specialist examination and, when necessary, molecular testing.

• Early berry symptoms are yellowish, greenish-gold or water-soaked spots that enlarge rapidly and may develop brown, reddish-yellow, purple or blue-gray centers.
• Berries soften, collapse, shrivel and either fall from the cluster or remain attached as mummies.
• Numerous small brown, violet, gray or gray-white pycnidia develop on the berry surface; the pale appearance is associated with cuticle separation and mature pycnidia.
• Pedicels and rachises may develop brown lesions and wilt, causing partial or complete bunch drying.
• Leaves may show irregular or zonate brown lesions, sometimes with pycnidia in humid conditions. Green shoots may develop elongated, depressed brown lesions or cankers, particularly near nodes.

The fungus survives in infected canes, rachises, dried stems and mummified berries. Pycnidia release conidia when wetted, and rain or overhead irrigation splashes spores onto susceptible tissues. Hail, berry cracking, insect feeding, bird damage, pruning wounds and other injuries greatly increase infection risk, although uninjured berries can also be infected under favorable conditions.

Dense canopies, poor air movement, frequent summer rain, prolonged surface wetness and fruit positioned close to contaminated soil increase disease pressure. The disease is especially associated with vineyards experiencing hailstorms followed by warm, humid weather.

Coniella diplodiella overwinters as mycelium or pycnidia in infected grape tissue, including mummified berries and canes. During wet periods, pycnidia produce and exude conidia through their openings. Splashing rain distributes the spores to berries, clusters, leaves and shoots.

Conidia germinate on wet plant surfaces and enter through wounds, microcracks, the pedicel or rachis, and sometimes directly through intact tissue. Lesions may appear within several days under warm conditions, while incubation is generally longer on unwounded berries or at cooler temperatures. New pycnidia develop on diseased tissue and provide additional inoculum during the season.

Remove badly affected clusters and prune out infected canes when practical, especially when disease is localized. Bag or otherwise contain removed material so spores are not redistributed during wet weather. Improve canopy ventilation and correct irrigation practices immediately.

After hail or severe berry injury, intensify scouting during the following wet, warm period. Distinguish white rot from black rot, Botrytis bunch rot, sour rot and other cane or fruit rots before choosing a control program. In commercial vineyards, disease-management decisions should be based on local extension guidance, cultivar, disease history, weather and the product label.

Use sanitation, dry-canopy management, removal of inoculum, careful pruning and prevention of berry injury as the core low-input strategy. Composting or disposal methods should prevent surviving fungal tissue from remaining near vines. Biological products containing antagonistic microorganisms may be available for grape diseases, but efficacy against C. diplodiella specifically is variable and often supported more strongly by laboratory or detached-berry studies than by U.S. vineyard trials; use them only when the product is registered and its label includes the intended crop and disease.

• Remove and destroy mummified berries, diseased clusters and infected prunings during the season and after harvest; do not leave them beneath the vines.
• Prune and train vines to improve air movement and sunlight penetration. Use appropriate shoot thinning, leaf removal and cluster-zone management without causing excessive sunburn.
• Use drip irrigation or another method that keeps clusters and foliage dry; avoid overhead irrigation.
• Minimize berry injury by managing hail risk where practical and controlling insects, birds and other causes of cracking or wounds.
• Avoid excessive nitrogen and excessive vine vigor, which can create dense, slow-drying canopies.
• Do not work vines when they are wet, and sanitize tools when moving from diseased to healthy blocks.
• Where reliable local trial information exists, select cultivars or rootstocks with better tolerance, recognizing that susceptibility varies by cultivar and disease complex.

• Grape white rot is often called hail disease because outbreaks frequently follow hail, but hail is not necessary for infection.
• The disease name refers to the whitish appearance of mature infected berries, caused by pycnidia and separation of the berry cuticle from underlying epidermal tissue.
• Pycnidia in infected grape residues can remain viable for many years, making postharvest sanitation important.
• Older publications may call the pathogen Pilidiella diplodiella or Coniothyrium diplodiella; modern taxonomic treatments generally place the species in Coniella.
• Coniella vitis and other related taxa can cause similar grape white-rot symptoms, so visual diagnosis alone may not resolve the species.