Dealing with Early Bunch Rot in Grapevines

Botrytis can infect berries at or shortly after bloom and remain symptomless until berries start to mature.  However, if berries are damaged, these latent infections can be activated and start to grow and sporulate.  This has happened in the past when a hailstorm has come through and caused berry splitting.  However, the infections that are developing this year are not always associated with hail damage.  What is different about this year is the amount of rain we’ve been getting.  Berries swelled rapidly resulting in squeezing and splitting.  Tight clusters also mean it’s much easier for Botrytis to grow from one berry to the next.  In addition, there is research to support increased water content of tissues can also activate latent infections.  Some work done by Dr. Helen Fisher and David Percival demonstrated that the waxy cuticle on the sides of berries that were tightly packed was thinner so it’s possible that the Botrytis is taking advantage of this to move through neighboring berries.

Frequently the bunch rot is starting from the centre of the cluster where squeeze is most likely to occur.  The high humidity and frequent rains we’ve experienced this summer, along with vigorous canopies complete the perfect storm for bunch rot to develop as Botrytis requires high humidity to sporulate.

Control measures

Sirocco is labeled for bunch rot suppression.  It works by desiccating the fungus as well as infected berries.  Several dry formulation fungicides, such as sulphurs and Folpan, may have similar activity.  Apply to the fruiting zone.  Then follow up the next day with a product with Botrytis activity, such as Elevate, Kenja, Scala or Switch(Botrytis-specific) or Luna Tranquility, Miravis Prime, Inspire Super, Diplomat or Timorex Gold which can be applied either to the fruiting zone specifically for bunch rot or full canopy for powdery mildew and Botrytis.  Double Nickel, Fracture/ProBLAD PLUS, Serenade OPTI, Serifel and Botector list suppression of bunch rot on their labels as well as some activity vs powdery mildew.  Make sure that you consider fungicide group rotations and avoid using products from the same group in back-to-back sprays.

Relative activity of Fungicides vs Botrytis bunch rot and Powdery mildew

Fungicide	Group	Botrytis	Powdery mildew
Elevate	17	3	0
Kenja	7	3	0
Scala	9	3	0
Intuity	11	3	1
Switch	12	3	0
Inspire Super	3+7	3	3
Luna Tranquility	7+9	3	3
Miravis Prime	7+12	3	3
Timorex Gold*	46	2	2
Diplomat	19	2	2
Botector*	NC	1	0
Double Nickel*	BM2	1	1
Fracture/ProBLAD	BM1	1	1
Serenade OPTI*	BM2	2	2
Serifel*	BM2	2	2