With the start of veraison comes the threat of bunch rot. And that can set the stage for sour rot. Controlling sour rot can be a difficult challenge, especially for grape growers in the central and southern regions of the San Joaquin Valley.

As berries begin to ripen and sugar content exceeds 8 percent, fruit injured by insects, such as thrips and omnivorous leaf roller (OLR), bird feeding, sun burn, mechanical or growth cracks, or lesions resulting from powdery mildew or black measles (esca) becomes increasingly susceptible to invasion by a wide variety of naturally occurring fungi.

The resulting rot can be severe as it progresses beyond the original injury, say UC Cooperative Extension IPM specialists. Masses of black, brown or green spores develop on the surface of infected berries. Bunch rot often leads to sour rot, which is caused by various microorganisms. They include acetic acid bacteria. Spread by vinegar flies attracted to the rotting clusters, these bacteria produce the characteristic sour smell of this disease.

The fungi and bacteria that cause sour rot are always present in vineyards, reports Allison Ferry-Abee, UC Cooperative Extension viticulture farm advisor for Tulare and Kings Counties, in the June/July issue of Vit Tips, UCCE’s San Joaquin Valley viticulture newsletter

Development of sour rot requires berries to be over 8⁰ Brix, and either free moisture must be present on the berries for several hours, or they must have wounds.

Most sour rot control strategies are more about reducing berry injury than controlling the fungi and bacteria themselves, Ferry-Abee notes.

In addition to controlling powdery mildew, thrips and OLR, she offers several more tips for significantly reducing the risk of sour rot disease

 For details on specific pesticide recommendations, see the UC IPM website (http://www.ipm.ucdavis.edu/).