Botrytis Bunch Rot Botrytis cinerea Dollars and Sense January 17, 2007 Santa Rosa, CA W. D. Gubler Department of Plant Pathology University of California, Davis

Disease Triangle ► Pathogen- Botrytis cinerea ► Environmental Conditions ► Host ► Time

Epidemiology ► Fungus is ubiquitous ► Fungus is a weak pathogen ► Attacks juvenile tissue, highly succulent and senescent tissue such as blossom parts, ripe fruit and injured or dead tissues. ► After infection the fungus can survive as a saprophyte on dead tissue or can produce sclerotia (resistant resting structure).

Epidemiology ► The fungus can be associated with debris on the vine or on the vineyard floor:  Sclerotia survive on canes or in fruit mummies.  Blossom debris ► Sclerotia can survive adverse conditions. ► Sclerotia germinate to produce conidia. ► Conidial production favored by successive interrupted wet periods. ► Conidia are dry and are dispersed by air currents or by water-splashing.

Epidemiology ► Infection occurs in dead and senescing flower parts and invades through the stamens into the flower receptacle. ► Pathogen can also invade the necrotic areas around the abscission layer of the shredded calyptra on the receptacle. ► Young berries generally resistant to infection due to epicuticular wax and chemicals in the wax.

Why Disease?? ► An outbreak of Botrytis bunch rot between véraison and harvest is regulated by a complex set of factors.  Of these, conidia dispersed in bunches and latent infection established at the berry base or receptacle during early season, and a stress factor(s), are the main factors responsible for symptom expression at véraison. ► Climatic conditions such as high humidity, dew and intermittent rain during late season will regulate bunch rot development.

Botrytis cinerea ► The fungus always attempts to penetrate sound berries directly through the skin, but most of these penetrations are unsuccessful. ► Skin damage (wind, birds, insects) thus also acts as a stress factor which offers another major pathway for the fungus to enter berries. ► In the event of wounding, a combination of fresh wounds, freshly dispersed conidia and free water on the berry surface is necessary for successful wound infection. ► A synchronization and combination of these events may not commonly occur in the vineyard.

Symptoms ► Can occur as 2 epidemics  Early mostly asymptomatic and occurring from pre- bloom to fruit set. ► Brown-reddish lesions develop on leaves. Shoot dieback. ► Blossoms become latently infected  However, not all infected blossoms result in disease  Later from pea-sized berries through harvest ► Starts as browning of the skin of the fruit and invades other berries through cracks and wounds.  Berry leakage ► Dry weather=lesions cease expansion ► Wet weather=lesions continue to expand and fungus moves to other berries thru spore movement or mycelial growth

Symptoms ► Poorly hardened shoots may show bleaching in the fall with development of black sclerotia forming in bleached tissue. ► This occurs mostly around nodes and suggests entry of the fungus through the petiole of a diseased leaf or through a leaf scar. ► Sclerotia in mummified berries.

Symptoms ► Newly grafted grapevine cuttings held in high humidity and at temperatures of 30 C can be infected and quickly destroyed by Botrytis cinerea. ► The fungus also may develop under the paraffin used to seal the graft union of grafted vines and inhibits development of the graft union.

Control of Botrytis Bunch Rot ► Fungicides ► Leaf removal ► Decreased fertilizer ► Cluster Architecture ► Trellising

Fungicides ► Timing  Bloom, Preclose, Veraison, Preharvest ► Application  Full coverage, Both sides of vine, Preventative ► Phenology  Blossoms, young fruit, older fruit, leaves, shoots ► Weather  Rain

Table 1. Results of Grape (J. riesling) bunch rot fungicide trial in Philo, Fungicides were applied at bloom ‘A’, bunch close ‘B’, veraison ‘C’, and 1 week preharvest ‘D’. 1 Severity is the percent of each cluster affected by Botrytis rot 2 Incidence is the percent of clusters with any rot. 3 Values in a column followed by the same letter are not significantly different according to Fisher’s LSD t test at P=0.05.

Canopy Management ► Leaf Removal  Effect on fungicide application ► Increases target site hit by 200x ► Timing=berry set  Removes dead blossom debris, results in thicker wax layer  Later leaf removal results in sunburn, reduced epicuticular wax thickness ► Hedging  Early Delays maturity  Late results in sunburn, Greenberry (Cladosporium cladosporioides) ► Crown Suckering  Only small reduction in disease ► VSP  Excellent Botrytis control if clusters are exposed

Other Cultural Practices ► If caught w/o fungicide application and can get in the vineyard  Use spray rig to spray high pressure directed air at clusters ► Interrupts infection process ► Must be done within 12 hours ► If you don’t leaf remove  Use leaf blower-type directed wind to remove blossom debris early in season

Cluster Architecture ► Tight clusters more prone to rot  Berry touch reduces the epicuticular wax layer on the berry = easier access by pathogen  Extend rachis??? Gib, DMI fungicides ► Reduce touching  Early exposure to sunlight ► Increase wax thickness

Fertilizers ► Use of fertilizer can increase vegetativeness  Increase shade ► Reduces wax thickness  Increase RH ► Reduces wax thickness  Decrease Temperature ► Increases favorable conditions for pathogen  Reduce wind speed ► Increases favorable conditions for pathogen  Increase succulence of tissue ► Easier infection of tissue

What Happened in 2006 ► Rain during bloom ► Rain preharvest ► Hang time  Rachis began to senesce ► Rachis infected resulted in berry drop  Ripe fruit + water = Rot  Sound berries less effected

What to expect in 2007 ► Botrytis epidemics cyclic  3 year cycle  2007??? ► Longer fruit exposed on the vine the bigger the chance of seeing disease ► Early leafing ► Fungicides if needed but always as a preventative treatment  Coverage!!! Best timing for good coverage is pre-close