History is never wrong. It’s whether paying attention to it is the right thing to do.

For California almond growers, looking into recent past seasons can offer plenty about the disease propensity of an orchard today.

If leaves are falling prematurely in late summer or early fall, chances are it is due to two diseases, scab and rust, according to University of California Cooperative Extension Farm Advisor David Doll who is based in Merced County.

Doll joined UC IPM specialist Walt Bentley recently for a foggy morning almond pest management field meeting at Rushing Ranch just west of Fresno, Calif.

It was sponsored by the San Joaquin Valley Sustainable Farming Project.

Scab, according to Doll, is the more insidious of the two diseases because it is a relative of alternaria, a disease common to the southern San Joaquin Valley. This relationship is a challenge to an almond disease control regime because treating both with similar fungicides can create cross-resistance.

Left untreated, scab and rust can cause pre-harvest defoliation and weaken trees, thus compromising future production.

Foliar diseases are becoming more problematic in California almond orchards. One reason is due to late, spring rains the past few seasons creating high orchard humidity. Another is the growing trend toward high density plantings and the almost constantly running micro sprinklers keeping orchards wetter longer.

“With the tighter spacings in orchards, it takes longer for the orchards to dry out in the middle of the day,” Doll said. “Just because the temperature increases outside the orchard, does not mean it is increasing inside.”

This 2012 dormant season has been one of the driest on record. Doll warns that a dry year may reduce scab and rust levels, but a wet season or high humidity in an orchard can cause the problems to persist or explode anew.

History plays a key role in whether a grower should treat for either disease. Premature in-season defoliation is one way to tell if there is a problem. In-season scouting can confirm the presence and severity of the diseases.

Rust appears as pale yellow-green spots, which turn bright yellow and become somewhat angular, in late spring on both leaf surfaces. Orange-brown pustules form in lesions on the lower surface of the leaf.

Scab overwinters as mycelium in twig lesions and sporulates on lesions beginning in late March. On leaves, the disease appears as indistinct, green-yellow lesions on the underside and, later, on both sides of the leaves. Once the fungus sporulates, the lesions turn olive green.

On fruit, small green to olive-colored circular spots develop on the upper surface. The spots grow and darken as the disease develops.

If scab has evolved into a serious problem, Doll recommends first a delayed dormant application of copper and oil.

“Ten years ago, you would hear us tell you not to spray during dormancy,” said Doll. However, the increasing humidity issue is changing that recommendation.