Brown Rot Blossom Blight

Young blossom spurs and associated leaves collapse to form shoot blight. Gum may exude at the base of infected flowers. Cankers on blighted twigs have tan centers with dark margins. At high humidity, gray to tan spore masses form on diseased flower parts and twig cankers.

Flowers may become infected from pink bud to petal fall and are most susceptible when fully open. In almonds, stigma, anthers, and petals are all very susceptible to infection.

The fungus survives in twig cankers and on remaining diseased flower parts and spurs. Spore pads (sporodochia) appear on these structures in late winter to provide inoculum for spring infection. Spores are airborne or rain splashed. Infection is favored by rainy weather with temperatures in the mid-70s during bloom. It can also occur in dry years with high humidity.

Control is based upon protection of flower parts. Ideally, applications should be made for each cultivar according to its stage of bloom. Otherwise, determine timing by the bloom of the most seriously affected cultivar in the orchard.

Treat at pink bud (5–10% bloom) and full bloom. One application at full bloom is sufficient in most orchards in most years if there is low rainfall. If brown rot has been severe or in years of extended bloom accompanied by rainfall, a second or even third application near full bloom may be necessary.

Cultivars highly susceptible to brown rot include Drake, Winters, and Wood Colony; other cultivars such as Butte, Carmel, and Ne Plus Ultra are very susceptible.