Joe Connell, UC Cooperative Extension Advisor Emeritus, Butte Co.

Shothole.  The primary concern now with shothole disease is the potential for leaf infections that can lead to defoliation and yield loss.  Dr. Jim Adaskaveg and I did work years ago that led to an understanding of how to more effectively “pest manage” this fungus. We know that the first round of disease in the spring comes from overwintering spores produced on leaf lesions in the fall. Occasionally a few blossom infections are found on the jackets and a little shothole in spring is not a big deal.An epidemic outbreak of leaf infections is a big deal and can affect your bottom line.  Periods of 10 to 16 hours of continuous moisture are needed for shothole infection of leaves.  Therefore, the primary objective of shothole sprays is protection of the foliage from bloom until five weeks after petal fall.

To “pest manage” this disease you need to pay attention to what’s happening with any shothole lesions you monitor regularly on leaves. When it’s cool and rainy the tree’s response to the disease lesion is slow.  The lesions may persist in the leaf rather than shot-holing out.  Watch out if you see a spore producing sporodoccia (the little black dot in the center of the lesion) in a leaf lesion. Once spore producing lesions are present on leaves and it rains, the secondary in-season disease cycle begins. When large numbers of splash dispersed spores germinate throughout the tree during rainy weather a shothole epidemic can occur. This can be followed by defoliation and even nut drop when the outbreak is extreme.  If continued rain is forecast, prevent an epidemic shothole outbreak by having effective protective fungicide sprays in place to prevent the secondary epidemic.

If on the other hand, you see a few shothole lesions and it warms up and stays dry, the tree’s response is to wall off the lesion with a yellow halo before sporodoccia can form. When the lesions drop out before making spores, the tree wins and the potential for shothole spore production is gone, as is the potential for an epidemic outbreak.

Scab.  The primary concern for scab is the potential for an epidemic outbreak and nearly complete defoliation in June to early July that can affect future yield. Somewhat like shothole, we have the potential to “pest manage” treatments for scab depending on the status of overwintering twig lesions and the timing of spring rains compared to twig lesion sporulation.

Some varieties such as Carmel, Winters, and Peerless readily produce over-wintering twig lesions on green wood.

The fungus begins its seasonal cycle by producing black spores around the margins of twig lesions from roughly late March to mid-April, most often forming spores in about mid-April.

This disease can become a problem when rains continue after spore formation.  If rain occurs after twig lesions sporulate, an epidemic scab outbreak can be present by June.

So, keep an eye on the twig lesions.  If it’s raining before twig lesions form spores, scab won’t be a serious problem. Once spores have formed on the twig lesions and significant rain is forecast, an April spray application is important to prevent infections now and defoliation later.  While this disease is favored by protracted spring rains, sprays at five weeks after petal fall have generally controlled scab.

By June, scab infections that started in May will show up as yellowish spots on the under surface of leaves as light comes through the leaf.   Later, the lesions develop a gray, greasy appearance visible on the upper leaf surface as well, ultimately producing oily greyish black spots on leaves as they defoliate.  Severe infections result in partial to complete defoliation of almond trees in early to mid summer.

Our research has also shown that later May sprays will further help reduce the severity of scab.  If we get into a continued rainy pattern into late spring, later applications of fungicides will help prevent serious defoliation from developing. Scab infections can produce black spots or a grey haze on hulls but this has no effect on kernel quality.

Fungicides are primarily protective and must be applied with good coverage and must dry on the tree before rain occurs to be effective and to reduce the chance of resistance developing.  The “best” program for one orchard may not be the “best” for another… heed the diseases present in your orchard.  Different classes of materials should be rotated in a spray program to reduce the chances of resistance developing (and to reduce the loss of our currently effective materials).