In almond orchards: Weather conditions trigger insects, diseases

The weather conditions of 2006 have made the almond growing season very difficult. Fungicide sprays were difficult to time because of the prolonged bloom due to poor chilling accumulation.

The untimely fungicide sprays resulted in Green Fruit Rot (Jacket Rot). This disease was most common in the Butte variety. The warm winter and spring rains may have contributed to the invasion of the leaffooted bug which in some orchards caused severe losses. The most affected varieties were Aldrich, Fritz, Monterey, Butte and Sonora varieties. This insect appears to have moved out of the orchards however rust, scab and Lethal Phytophthora Canker are making their appearance at this time.

Lethal Phytophthora Canker is a disease that attacks and kills almond scaffold limbs. The main symptom is profuse gumming on the scaffolds around the head (crotch) of the tree.

The gumming can be so abundant that it will drip to the ground. Research has shown that the fertilizer Nutri-Phite can suppress canker development when applied in the fall or in the spring.

Almond rust has become more common in almond orchards. It is known as a late season disease because it appears in almond orchards in early to late June. The first symptoms are small yellow spots on the upper surface of the leaves. On the lower surface of the leaves, these spots take on a rusty red colored appearance which is due to the eruption of spores from the lesion. The spores are spread by air movement and infect other leaves. Young twigs may be infected but twig lesions are seldom seen in almonds.

Fungus rust development depends on high humidity. Rain in late spring and early summer create ideal conditions for disease development. When the disease spreads to most of the leaves on a tree, it can lead to premature defoliation. This can reduce next year’s bloom. The rust fungus survive from one season to the next in infected leaves and possibly infected twigs.

Fungicide application for the control of rust should be done before symptoms appear on the leaves. In orchards with a history of rust, two or three fungicide applications may be necessary to control leaf infections.

The best times to treat is five weeks after petal fall and in the summer. Fungicides registered for rust control are wettable sulfur, sulfur dust, Abound, Flint, and Maneb. Both Abound and Flint belong to the same chemical class. They must be rotated with Maneb, sulfur or wettable sulfur to prevent resistance.

Almond scab, a common disease in the Sacramento Valley, can be found in many orchards in Kern County. High humidity is required for scab development and tree defoliation will occur at high levels of infestation. The most susceptible varieties are Carmel, Padre and Nonpareil.

The first symptoms appeared late June in the form of black-grayish spots underneath the leaf. Later on these spots change to a darker black color and appear on hulls and on this year’s shoot growth.

Scab can be controlled with shothole disease sprays but if rains occur in mid- to late spring an additional spray may be needed. A good time to treat is five weeks after petal fall. However, a treatment two weeks after petal fall may provide a more complete control.

The fungicides registered for scab control are lime sulfur in the winter include Abound, Pristine, Flint, Captan, Maneb, Ziram and Topsin-M. For resistance management rotate these fungicides and please read and follow label directions.

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