Following years of drought, Northern and Central California have been experiencing a deluge of rain. Continued rains have hindered field work and agricultural operations. In addition to the stress caused to the plants due to water logging conditions, fungus diseases that favor wet and rain conditions are a major concern to the growers. In the San Joaquin Valley, many of the almond orchards are at 20 to 80% bloom and are prone to fungal infections. Therefore, it is important to the identify the diseases spread by rain during bloom and plan the sprays targeting these pathogens.
Major Diseases of Almonds
Caused by Colletotrichum acutatum or C. gloeosporioides. Symptoms include yellow to orange colored sunken lesions on leaves and fruits. Infected nuts often gum profusely. Fungus destroys the nut meat, wood and branches, causing limb dieback and long term damage to the crop. Warm and rainy weather is ideal for the fungus growth. All varieties are susceptible to the disease.
Brown Rot: –
Most damaging disease of almonds during rain at bloom. Caused by Fungus Monilinia laxa, M. fructicola. Grey to tan spore masses are visible on diseased parts. Cankers and gummosis is observed on infected twigs. Brown rot fungus attacks the tree by invading the anthers and pistils, moving into the shoot. Spores are rain-splashed and infect the plants by entering the open flower. Butte and Carmel are most susceptible to Brown rot. Ne Plus Ultra and Mission are moderately susceptible. Nonpareil and Peerless are the least susceptible to brown rot.
Jacket Rot: –
Also called Green Fruit Rot, caused by Botrytis Cinerea, Sclerotinia sclerotiorum, Monilinia laxa. Symptoms develop during the latter part of the bloom period and include water-soaked brown lesions developed on petals. Cool and wet conditions increase the risk of Jacket Rot. Butte, Ne plus Ultra, Merced, Carmel, Price and Nonpareil are all susceptible.
Caused by Cladosporium carpophilum. Symptoms include grayish-black spots on fruit, leaves and green shoots. Causes premature leaf drop. Spores are spread by wind and rain. All cultivars are susceptible.
Shot hole: –
Caused by Wilsonomyces carpophilus. Symptoms include reddish specks, lesions on leaves and fruit. Spots fall out of the leaves causing a shot hole. Disease causes defoliation. Rain increases the spread and intensity of the disease.
Leaf Blight: –
Caused by Seimatosporium lichenicola. Fungus attacks the base of leaf petioles causing the leaves to brown and die. Fungus favors wet spring weather. Leaf blight is common in Northern California.
Leaf Rust: –
Caused by Tranzschelia discolor. Symptoms include yellow spots on upper leaf surface and reddish brown colored lesions on lower leaf surfaces. Causes defoliation. Fungus favors high humidity and rains.
Leaf Spot: –
Caused by Alternaria alternata on Almonds. Symptoms on leaves appear in late summer as large brown, circular lesions with black center. Pathogen causes debilitating defoliation in Southern San Joaquin Valley. Almond Varieties Carmel, Nonpareil, Butte, Price, Sonora, Mission, and Peerless are all affected by the pathogen. Standing Rain or dew is ideal for disease development.
Crown, Collar and Root rot: –
Caused by Phytophthora sp. Symptoms include brownish discoloration on the root or crown tissue. Disease is severe in young almond trees. Poor drainage, standing water around crown region favor Phytophthora infections. Plum root stocks are more resistant to the infection than peach or peach-almond rootstocks.
Disease testing: –
Favorable weather conditions such as rain during bloom will increase the incidence, spread and intensity of these diseases. Therefore, almond growers are advised to look out for the symptoms and send any suspected plant material and soil to SunBurst lab for testing, followed by the control of pathogen through fungicide sprays.
Sources: – Disease Identification Guide for Almonds and Pistachios by Syngenta, Rain-at-bloom almond disease threat by Western Farm Press