Know your grapevine bunch rots

Page last updated: Tuesday, 14 May 2019 - 1:10pm

Bunch rots of grapes are widespread in Western Australia and reduce yield and quality. Weather conditions and control strategies influence the severity of losses which can vary between vineyards. These bunch rots can affect both wine and table grapes and control options differ depending on location, variety and trellising.

Bunch rots can be caused by several different fungi, many of which produce similar symptoms on the berries. Confusion over the cause of the bunch rot can lead to poor management decisions and crop failures.

Accurately identifying the cause of a bunch rot on your vines is critical so the correct control strategy can be implemented.

Bunch rots can affect both wine and table grapes and control options differ depending on location, variety and trellising. If you suspect a non-botrytis bunch rot is the problem in your vines, samples can be sent to DDLS Seed Testing and Certification for diagnosis.

Botrytis bunch rot

Botrytis is the most common bunch rot found in WA and the most damaging. It is caused by the fungus Botrytis cinerea. Bunch rots caused by other fungi are often attributed to botrytis due to its prevalence.

Grapes infected with Botrytis bunch rot show grey fungal growth
Grapes infected with Botrytis bunch rot showing grey fungal growth

Botrytis infection can occur at flowering although symptoms may not be obvious until close to harvest. Berry symptoms initially appear as water-soaked spots where the skin easily slips off the berry — known as slip skin. After infection, the berry changes colour and grey fungal growth appears. Botrytis spreads easily to other berries as the fungus sporulates. Berries may shrivel and become mummified if hot conditions occur after infection. Numerous chemicals are registered for its control and cultural techniques minimise loss. For more information refer to Botrytis in wine grapes in Western Australia.

Aspergillus

Aspergillus rot is common on mature berries and is generally considered a secondary invader. However, it is known to infect intact berries directly through their skins during warm temperatures (20–30°C) when water is present. Infected berries are initially tan to brown, but are soon covered with masses of brown or black spores.

Grapes infected with Aspergillus rot
Grapes infected with Aspergillus rot

Bitter rot

Bitter rot is caused by the fungus Greeneria uvicola and is usually only associated with ripe fruit. Infection occurs at flowering but remains latent until berry maturity. White berries turn brown and concentric rings of spore bodies eventually cover the entire affected berry. Berries soften and detach easily from the bunch. Optimum infection occurs at 28–30°C.

Chardonnay grapes infected with the fungus Greeneria uvicola which causes Bitter rot
Chardonnay grapes infected with the fungus Greeneria uvicola which causes bitter rot. Photograph courtesy of Suren Samuelian, Charles Sturt University.

Bitter rot is not known to occur in WA and suspected infections should be reported to the Pest and Disease Information Service (PADIS) on 1800 084 881.

Black spot

Commonly referred to as anthracnose, this is a wet weather disease that occurs sporadically in WA. It is generally more visible as a foliar infection but on berries it produces circular lesions that have black margins which turn white with age.

Grape leaf showing lesions characteristic of black spot infection
Grape leaf showing lesions characteristic of black spot infection

Botryosphaeria bunch rot

Commonly associated with trunk disease symptoms, Botryosphaeria species are known to cause bunch rot and have been found in all growing regions of WA. They are generally only seen close to harvest and disease progression is rapid. Initial symptoms look similar to botrytis bunch rot, with water-soaked berries that change colour. As the infection progresses, small black pimples appear on the berry surface before the berry shrivels, possibly falling from the stalk.

Grapes infected with the fungus Botryosphaeria which causes bunch rot and is commonly associated with trunk disease
Grapes infected with the Botryosphaeria fungus which causes bunch rot and is commonly associated with trunk disease

Cladosporium rot

Infected berries develop black, soft circular lesions which can cover almost the entire berry and produce olive velvet patches. Infection is favoured by delayed harvest or berries with high phenolic maturity. Berry damage is required for infection.

Berries infected with Cladosporium rot develop black, soft circular lesions which can cover almost the entire berry
Berries infected with Cladosporium rot develop black, soft circular lesions which can cover almost the entire berry

Downy mildew

Berries are susceptible to downy mildew infection until they are about pea size but the bunch stem and berry stem remain susceptible. White down can be seen on recently infected tissue. After infection, the tissue stops growing, turns brown, shrivels and dies.

Berries showing characteristic symptoms of Downy mildew infection
Characteristic symptoms of downy mildew infection

Penicillium

This secondary disease is characterised by berries covered in a mass of coloured spores, usually blue or green. Infection is more obvious on mature berries and can be associated with a musty or mouldy odour.

Penicillium is a secondary disease and characterised by berries covered in a mass of coloured spores
Penicillium is a secondary diseasecharacterised by berries covered in a mass of coloured spores

Phomopsis bunch rot

This rot is associated with distinctive small black leaf spots with yellow halos and black lesions on the lower sections of canes. Phomopsis viticola berry infection is rarely seen in eastern Australia but is favoured by prolonged spring rainfall. It can be confused with Botryosphaeria rot.

Grape leaf showing the characteristic leaf spots with the yellow halo
Grape leaf showing characteristic leaf spots with yellow halo

        

Grape stem showing lesions characteristic of Phomopsis infection
Grape stem showing lesions characteristic of Phomopsis infection

Phomopsis viticola is not known to occur in WA and suspected infections should be reported to the Pest and Disease Information Service (PADIS) on 1800 084 881.

Powdery mildew

Although not recognised as a bunch rot, powdery mildew can infect immature berries between flowering and up to four weeks later. Severe infection can scar the berry surface. As the berry ripens, the scar cracks and splits, leading to infection from other bunch rots such as Botrytis.

Powdery mildew infecting immature berries
Powdery mildew infecting immature berries

Rhizopus species

Numerous white fungal stalks with small circular spore heads develop from berries infected with Rhizopus species. Infected berries become soft, brown and ooze juice to adjacent berries, spreading the infection.

Ripe rot

Caused by different species of Colletotrichum, this bunch rot is usually associated with warm weather (above 25°C). Infected berries develop a reddish brown circular spot at the infection site that eventually covers the whole berry. Characteristic salmon or orange-coloured spore masses are produced on the rotting berry. This is rarely seen in WA, despite the causal fungi being recorded here.

Berries infected with a range of species of Colletotrichum develop a reddish brown circular spot at the infection site that eventually covers the whole berry
Berries infected with a range of Colletotrichum species develop a reddish brown circular spot at the infection site that eventually covers the whole berry (courtesy Chris Steel, Charles Sturt University)

Sour rot

Sour rot is caused by a mixture of fungi, yeast and bacteria. It is characterised by a vinegar smell and the presence of vinegar flies, the larvae of which help spread the rot throughout the bunch.

Sour rot begins as a secondary invader but juice from affected berries can drip onto nearby berries, causing cracks in the skin and infecting others.

Sour rot is caused by a mixture of fungi, yeast and bacteria and is characterised by a vinegar smell
Sour rot is caused by a mixture of fungi, yeast and bacteria and is characterised by a vinegar smell

Contact information

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080