Pest & disease Botrytis

Late season Botrytis

There is nothing worse than seeing your beautiful bunches rotting in front of your eyes.

The presence of La Niña – with rain events scattered through the harvest period – could affect grape growers, with late Botrytis rearing its ugly head.

While it might be only one of many bunch rots, with another La Niña season currently predicted for next year, you might ask how can you guard your bunches from Botrytis bunch rot?

The answer, to a certain degree, lies in the way Botrytis infects.

Flowering is the first opportunity for infection to occur. Spores that have survived in bunch remnants (floral parts and leaf material) can be splashed around by rain, wind, or carried by insect vectors. Spore germination can occur at any time if conditions are favourable (temperatures around 18-21º Celsius) and if vines stay wet from rain, fog, dew or mist. Once tissue is infected, the infection can lie inactive – latent – until conditions are ideal for spread. Fortunately, not all latent infection leads to rotten berries, as hot, dry weather through summer and autumn can prevent development. High relative humidity in the bunch zone and excessive soil moisture can activate Botrytis.

Botrytis spores are almost always present in vineyards. Late season infection often occurs when rain splits berries and spores enter the wound. Any other berry wounds – from insects, mildew infection, frost, hail or sunburn – can be entry points too. Preventing berry split reduces the risk of Botrytis. What can you do?

If there was a severe Botrytis infection in your vineyard this season and/or wet spring, the risk of Botrytis next season is high. Varieties with a high susceptibility to berry splitting will need extra attention.

Steps to take:

- Remove mummified bunches

- Reduce spore load by removing vine debris and remnants

- Apply protective fungicide at 80 per cent capfall – good coverage of flowery parts is critical

- If the flowering period is prolonged, protect with a fungicide before it rains, always paying attention to optimum coverage

- Apply a protective spray at prebunch closure, as this is the last opportunity to get good spray coverage within the bunch, where latent infection often emerges

- Minimise berry wounding (wounds are entry points) by controlling light brown apple moth (LBAM) and powdery mildew

- Limit humidity in the fruiting zone; manage canopies to allow air flow

- Bunch thinning, stretching and bunch removal helps prevent compact and crowded bunches

- Prevent excessive soil moisture

- It is difficult to get good spray coverage inside mature bunches, where Botrytis often emerges and spreads. Once botrytis appears in a bunch, spray application of sanitisers only inhibits the fungus on the surface at best, and even surface mould is killed, the infection remains. MRLs must also be considered if you need to treat Botrytis late season.

- There are new chemical options available for Botrytis control – talk to your chemical representative. v

Information for this article has been sourced and adapted from: https://www.wineaustralia.com/ getmedia/b78baaea-d1de-469f-a8aa1aec30d4d756/201307-Botrytis-QA?ext=.pdf https://www.wineaustralia.com/ getmedia/3e2d4ecc-3d70-41db-b2b00b897a513fb8/CORD_Factsheets_ BotrytisManagement