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Bloom Disease Control

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Wet spring weather spells trouble, usually, for almond trees. Rain and fog are often accompanied by higher levels of disease-causing inoculum, setting the stage for crop-damaging outbreaks of nasty organisms such as brown rot, shot hole, scab, rust, leaf spot, and the newly resurgent bad actor, anthracnose. The following tips on controlling bloom period diseases were gleaned from the writings of Dr. Beth Teviotdale, UC Extension plant pathologist, Kearney Agricultural Center, and from the UC Division of Agriculture and Natural Resources publication, IPM for Almonds. Most bloom diseases can be controlled or at least slowed down by well-timed applications of the appropriate spray material.

Brown Rot

The first disease to appear when trees emerge from dormancy, brown rot goes to work in late winter and early spring, producing fungus spores on old infections. The spores spread on the wind, infecting open flowers through their pistils and stamens. The first symptoms are withered blossoms, which remain on the tree covered with light brown, powdery masses of fungus spores. The infection moves from the blossom into adjacent twigs and kills them as well.

Most brown rot control programs begin with a treatment at late pink bud, with a second treatment at full bloom. Treatment at petal fall is least effective. Dormant treatments with currently approved materials are ineffective for brown rot.

At pink bud, select a fungicide that has some systemic activity, such as Benlate, Topsin, or Rovral. After the flowers are open, choose Rally, captan, or maneb.

Shot Hole

This disease causes lesions on leaves, flowers, fruit, and sometimes on shoots. Early in the season, small purplish areas develop on young leaves, turning to spots ranging from 1/8 to 3/8 inch in diameter. Later, leaf tissue in lesions on young leaves dies, and may fall out as the tissue around it grows away from the dead tissue, leaving a "shot hole."

Shot hole can defoliate trees, and severe infections can cause fruit drop or gummy, deformed nutmeats.

Shot hole overwinters in infected buds and twigs. Warm temperatures and free moisture get it going again in spring. If extended spring rains occur, the infection can become serious.

If shot hole has been a problem, a treatment no later than late petal fall is recommended. Once shot hole is found in an orchard in spring, additional applications are needed if rains that last several hours continue. Captan, maneb, ziram, and Rovral are considered effective for shot hole.


The scab fungus overwinters in lesions on infected twigs and resumes growth in March. It is spread by spring rains and sprinkler irrigation. Scab is worse in orchards with high relative humidity.

Leaf and fruit lesions usually do not appear until June or July. This pest, which has become more prevalent in recent years, can cause severe early defoliation.

Dormant applications of liquid lime sulfur followed by one or two treatments with Benlate, Topsin, captan, maneb, or ziram usually provide good control.

Leaf Spot

Altenaria leaf spot is most common in the southern San Joaquin Valley, but has recently been reported in the Sacramento Valley as well. Dense, humid orchards are most likely to become infected with the disease. Leaf spot causes large lesions on leaves in summer and can strip a tree of leaves before harvest. Little is known about it. Rovral gave the best control in the only test completed thus far.


This disease has the potential for severe damage in a short period of time. The pathogen attacks flowers, fruit, and leaves. Infected fruit gum profusely and turn orange. The fungus requires water for reproduction, dispersal, and infection. Work on the epidemiology and control of this disease only recently began.


Hundreds of tiny reddish brown pustules erupting on the underside of leaves in late spring or early summer signal an infestation of rust. Infected leaves drop off. Defoliation from rust can be severe. The disease is controlled by one or two applications of sulfur in late spring and early summer.

More Tips

Be certain of your target, Dr. Teviotdale cautions. "It is crucial to determine which diseases are present in your orchard, which are of greatest concern, and to create a control program based on this information."

Dr. Teviotdale notes that is best to avoid treatments with any one fungicide. "Multiple applications with only one material invite trouble with resistance," she advises. "By including alternative materials, you have a better chance of protecting your orchard from falling victim to a previously overlooked disease."

Timing can be a difficult issue, she acknowledges, but when in doubt, "try to make your application before the rain." Lots of rain and moderate temperatures create an ideal environment for most plant diseases to multiply and spread, she adds.

Application techniques are of crucial importance. Ground application is generally regarded as the most effective, but it must be done correctly. Use properly calibrated and directed nozzles, and maintain a slow speed. Remember: Disease control is only as good as the coverage!