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Botrytis bunch rot another challenge ahead

Viticulture
17.11.2020
Vine Talk columnist and Syngenta Technical Services Lead Dr Brandy (Belinda) Rawnsley.
Have you heard the latest tips, tricks and industry advice? Vine Talk author, Dr Brandy (Belinda) Rawnsley takes a look at the issues facing grape growers.

By Brandy Rawnsley

Last month we talked about the risk of downy mildew in a wetter than average season. Now it’s time to consider botrytis grey mould and how to minimise the risk of infection.

Botrytis cinerea is a fungus that requires moisture and a wound site to infect. Although we commonly associate the disease with bunch rot later in the season, infection can take place much earlier.

In fact, flowering is the key time when botrytis infection begins. At capfall, the fungus uses the wound site to invade the tissue but remains unseen (dormant) as green hard berries don’t allow colonisation. The risk of botrytis infection increases when there is an extended flowering period and excessive moisture.

Symptoms of botrytis often don’t become apparent until the berries start to ripen. As berries ripen, sugars stimulate growth of the fungus that invaded the tissue back at flowering.

Infection can also occur once berries ripen, especially those that are wounded by insects, birds, mildew or berry split caused by rain or hail.

Spores germinate in the presence of moisture when temperatures are around 18-21°C. Botrytis appears as grey fluffy mould. Botrytis can spread easily to other berries and before you know it, the whole bunch is infected.

Weather conditions are likely to be wetter for longer this summer. As botrytis needs water for germination, the risk of disease developing is higher than normal.

A protective fungicide, such as SWITCH applied at 80% capfall (E-L 25) when most of the cap scars are exposed, is a great way to prevent early infection. If you’ve missed this spray, the next critical time is pre-bunch closure (E-L 31). This ensures the fungicide can cover the berry and penetrate the bunch. Big, tight bunches are going to be more prone to infection.

But don’t only think of fungicides for botrytis control. Wounding increases the risk of botrytis, so targeted insecticide applications are strongly recommended to guard berries from insect feeding damage. VOLIAM TARGO offers control of numerous insect pests and mites on wine grapes up to E-L 29.  PROCLAIM OPTI can be used until E-L 31 for targeted control of key lepidopteran pests.

Dr Belinda (Brandy) Rawnsley is a Syngenta Technical Services Lead for Viticulture and Horticulture.