Better Yields Through Science

Almond Diseases and Pests

DiseaseSymptoms, Life-cycle and Pest Management
Powdery MildewPathogens: Podosphaera (=Sphaerotheca) pannosa, Podosphaera tridactyla, and P. leucotricha

Typical symptoms of powdery mildew include russeting on almond hulls. The symptoms are reminiscent of rusty spot on peach fruit caused by Podosphaera leucotricha but without the typical powdery white growth. No conidia (asexual spores) or chasmothecia (sexual fruiting structures of powdery mildew fungi) are generally observed on the fruit. Foliar and twig symptoms are absent.
San Jose ScaleScientific name: Diaspidiotus (=Quadraspidiotus) perniciosus

Scales suck plant juices from twigs and limbs, and inject a toxin, resulting in loss of tree vigor, growth and productivity, and death of limbs. A red halo is produced around a feeding site on 1-year-old green wood. Untreated infestations can kill fruit spurs and scaffold wood within 1 to 3 years. Dormant season treatments are the key to controlling this pest.
Shot Hole or Coryneum Blight Pathogen: Stigmina carpophila, syn. Coryneum carpophilum or Wilsonomyces carpophilus

Spots occur on leaves, fruit, twigs, and flowers; however, flower and twig lesions are relatively scarce or difficult to find. Leaf lesions begin as tiny reddish specks that enlarge by several millimeters into spots having tan centers and purplish margins. When the fungus sporulates, the fruiting structure appears as a small dark speck (the sporodochium and spores) in the center of the spot; this is a diagnostic characteristic of shot hole disease. Spots on young leaves usually fall out, leaving a hole (the shot hole); older leaves retain their lesions. Fruit spots are small with purplish margins, slightly corky and raised. Spots are found on the upper surface of fruit with respect to the way it hangs on trees. Heavy infection of young fruit may cause fruit drop or distortion and gumming of fruit.

The fungus survives on infected twigs and as spores in healthy buds. Spores are moved by water to new sites; prolonged periods of wetness, either due to rain or sprinkler irrigation, are required for the disease to develop. Shot hole can cause losses in yield, defoliation, and weakened trees.

Monitor orchards in fall and spring for shot hole lesions and fruiting structures. Fruiting structures appear in the center of leaf lesions as small black spots and can be seen with a hand lens. If fruiting structures are present in leaf lesions in fall, there is a high risk of shot hole development the following spring and a petal fall treatment should be applied. Whether or not a petal fall treatment is applied, monitor leaves in spring for lesions with fruiting structures. As soon as fruiting structures are evident, apply a treatment; continue treatments at the specified label interval as long as conditions are conducive to disease development.
ScabPathogen: Cladosporium carpophilum

Grayish black, soft looking spots form on leaves, fruit, and twigs. Young lesions are indistinct small yellow specks, best seen by holding a leaf up to the light. Lesions usually are not visible until late spring or early summer.

Scab may be controlled by shot hole sprays, though a delayed dormant spray for scab improves the efficacy of spring control programs. A scab treatment may be required if rain occurs into mid- to late spring.
Brown Rot Blossom BlightPathogen: Monilinia laxa; rarely Monilinia fructicola

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. Spores are airborne or rain splashed.

Treat at pink bud (5-10% bloom) and/or full bloom. One application at full bloom is sufficient in most orchards in most years if there is no rain. 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.
Alternaria Leaf SpotPathogen: Alternaria alternata

Alternaria leaf spot appears as fairly large brown spots on leaves, about 0.5 to 0.75 inches (12-18 mm) in diameter. The spots turn black as the fungus produces spores. Leaf spot develops most rapidly in June and July, and trees can be almost completely defoliated by early summer when the disease is severe. The disease appears to be most severe where dews form, humidity is high, and air is stagnant.

Monitor for signs of the disease in April through June. If monitoring indicates the presence of Alternaria, begin late spring treatments about mid-April. In orchards with a history of the disease, treat in mid- to late April and 2 to 3 weeks later.
RustPathogen: Tranzchelia discolor

Rust appears as small, yellow spots on the upper surface of leaves. On the lower surface of the leaf these spots take on a rusty red appearance when the rust-colored spores produced in the lesions erupt through the surface. These spores are spread by air movement and infect other leaves to continue the disease cycle. Young twigs may be infected, but twig lesions are seldom seen on almond. Rust occurs sporadically.

The development of rust is favored by humid conditions, and the disease becomes worse when rain occurs in late spring and summer. Trees can be defoliated quickly when rust becomes severe. The rust fungus survives from one season to the next in infected leaves and possibly also in infected twigs. To be effective, treatments must be applied before rust symptoms appear on leaves. Examine one-year-old fruiting wood for small blisters or longitudinal splits. If twig cankers are found and rain is forecasted, make a treatment. Fall and Dormant applications are efficacious in helping manage the disease.