Almond trees are susceptible to bloom and foliar diseases when it rains at bloom, and decisions of when to spray and what fungicides to use can be difficult.
In the San Joaquin Valley we are usually in a low precipitation region and we cannot predict when and how much it is going to rain. We often receive rain during bloom which can result in favorable conditions for several plant pathogenic fungi to cause spring time diseases of almonds.
The fungi that cause these diseases are usually always present in almond orchards, some times in higher or lower amounts depending on the previous year’s disease levels and current environmental conditions.
Fungicide control programs
Generally, a good disease control program is based upon a wise choice of fungicides and good timing and coverage. Growers should assess the diseases present in their orchards and select materials carefully.
Not all fungicides are equally effective on all diseases. It is a good idea to use more than one kind of fungicide for a broader spectrum of activity.
Usually two sprays are made for brown rot control. The first is usually done at 5 percent to 20 percent bloom using a systemic fungicide. Some of these fungicides may require a contact fungicide to reduce resistance. Resistance to these fungicides can develop over time and repeated use, thus try to rotate the fungicides you use.
The second spray should be done at about 80 percent to full bloom or two weeks after the first spray. This is the most effective brown rot spray. Depending on the weather, a third spray may be necessary if rains persist and two weeks of protection have gone by. Since we cannot predict the weather at bloom time, we must at least take some initial action to protect our crop.
Application techniques are also important. Usually ground application is better than air, but care must be taken that both are applied correctly. In general, use properly calibrated and directed nozzles and maintain a slow speed.
Brown rot and shot hole
Most orchards are treated at least once during bloom for brown rot. The brown rot fungus (Monilinia laxa) attacks the tree by invading the anthers and pistils of the flower when it is open. From there the fungus can move into and kill the spur or shoot.
Young fruit are also susceptible in early spring and infection of fruit may extend to and kill spurs and shoots. Although all cultivars of almond are susceptible to brown rot, they vary in their degree of susceptibility; Butte is the most susceptible variety followed by Carmel. Ne Plus Ultra and Mission are only moderately susceptible, while Nonpareil and Peerless are the least susceptible to brown rot.
Varieties that are susceptible to green rot or jacket rot (caused by Monilina laxa, Botrytis cinerea, and Sclerotinia sclerotiorum) are Butte, Ne Plus Ultra, Merced, Carmel, Price, or any variety with tight clusters. Nonpareil can be affected by this disease if the right environmental conditions occur.
The time of infection for green fruit rot or jacket rot is from flower opening to petal fall. Brown rot blossom blight is usually controlled with pink bud and full bloom treatments with the full bloom treatment the most important in my opinion.
A third petal fall spray may be necessary in years favorable to disease (rain). If bloom is strung out and the weather is wet and rainy, no more than 10 days should elapse between treatments.
The shot hole fungus (Wilsonomyces carpophilus) is notoriously more prevalent in wet years. This fungus requires water for all its activities, so periods of extended rainfall create a situation that favors shot hole disease epidemics.
The fungus can cause lesions on leaves and fruit, but most of the time it infects the leaves as they emerge from the leaf bud. Leaf infections lead to defoliation, which usually occurs in early spring. Shot hole infection of young fruit, shortly after they emerge from the jacket, can cause the fruit to drop. As fruits enlarge, shot hole infection results in a lesion, but the fruit no longer fall.
About the first of May, when the embryo of the nut begins to grow, the hull becomes resistant to infection and no further lesions develop. Shot hole is usually controlled by fungicide applications after bloom.
Until recently, scab (Cladosporium carpophilum) was considered more of a curiosity rather than a crop threatening disease — this has changed.
In the last several years more and more orchards have developed scab problems and the disease is often serious. The fungus causes greasy black spots on fruit, leaves, and green shoots. The shoot lesions are the overwintering sites for the fungus and the source of new spores in the spring. No apparent damage is done to the fruit, but the leaves fall. Scab can completely defoliate a tree in a short time.
Severe defoliation in early summer was even observed in several orchards in Madera County last year. All cultivars appear susceptible, but Carmel seems especially vulnerable.
Scab is controlled by fungicide applications from two to five weeks after bloom. Earlier treatments are not effective alone, but increase the protection provided by the later treatment. Applications later than five weeks after bloom are less effective, especially in the southern part of the San Joaquin Valley. Not all fungicides are effective against scab — thus it is important to include scab-active materials in an overall treatment program.
Rust (Tranzschelia discolor f. sp.dulcis) can also cause defoliation. Both rust and scab are favored by high humidity and usually are worse in years when late spring rains occur. Orchard culture that produces humid conditions for long periods is ideal for both diseases.
Like scab, rust usually appears in late spring or early summer. The fungus attacks leaves, but not fruit. On leaves, it produces small, bright yellow dots on the upper leaf surface, and reddish orange pustules on the lower leaf surface. The only material registered that has any efficacy is sulfur. One or two applications of sulfur in late spring can usually control rust.
Leaf spotStill another defoliating disease, Alternaria leaf spot, is also new to the San Joaquin Valley. Nothing is known about the disease except that it appears in early summer, causes large lesions on leaves, and can cause defoliation. Carmel, Nonpareil, Butte, Price, Sonora, Mission, and Peerless are affected. Sonora is somewhat more susceptible than the others. Leaf spot has been around for several years, and only at a few locations has it caused enough damage to be of concern. But be on the alert.
Rovral applied at five weeks after petal fall has some efficacy against Alternaria leaf spot, but does not prevent serious defoliation. The limitation of treatment no later than five weeks after petal fall may be partly responsible for the poor control.
The leaf blight fungus attacks the base of the leaf petiole and the bud that sits between the leaf and shoot. The leaf dies, turns a light tan color, and remains stuck to the tree. Later these leaves are then covered with the black growth of secondary fungi decomposing the infected leaves.
Leaf death is of less importance, however, than the bud death that accompanies it. Leaf blight is more common in Northern California and fortunately we see it seldom here in Madera County. Generally, scab and shot hole programs control leaf blight.
An extremely damaging fungal disease, Anthracnose (Colletotrichum gloeosporioides) was severe in some orchards in Northern California last year. Anthracnose has been in California for many years, but has become more severe recently, attacking leaves and fruit. Leaf lesions have a bleached appearance and can cause the leaves to drop, though defoliation is not usually very pronounced.
Anthracnose causes deep lesions; the affected area turns a rusty reddish brown, and older fruit often gum profusely. Inside, the nut meat is destroyed. The fungus is reported to invade the wood, and the branches with infected fruit weaken and die. In addition to destroying the crop, long term damage and weakening of the tree may occur.
Varietal differences in susceptibility are not clear. It appears that good scab control programs appear to provide some control against this disease. Orchards which have a history of Anthracnose should be treated during bloom, preferably at pink bud, to help reduce inoculum buildup as much as to protect blossoms. Trees should be protected before every rain, thus repeated applications may be necessary through spring.
The fungicide efficacy and timing for deciduous tree fruit and nut crops and grapevines can be found at www.imp.davis.edu.
For the tables associated with this article, visit http://cemadera.ucdavis.edu/newsletterfiles/The_Pomology_Post16111.pdf.