A summer farm call to an orchard close to the Merced River yielded my first confirmed case of almond leaf scorch. This orchard consisted of a 12th leaf planting of Monterrey, Nonpareil, Carmel, and Sonora all on Nemaguard rootstock, planted in the Snelling Sandy Loam soil series, and irrigated by microsprinklers.
The orchard looked relatively healthy from afar, but a notable discoloring of the leaves was seen in the Sonora variety. A closer inspection revealed a large percentage of trees (i.e. 80%) that appeared to be infected with of almond leaf scorch. Some Nonpareil trees were symptomatic, but very few of the other pollinators (i.e. Monterrey and Carmel) had symptoms.
Picture 1: A tree severely affected with almond leaf scorch. Trees "bronze" over in late summer. This picture was not from the farm call listed above.
Upon seeing the symptomatic leaves, it became clear that Almond Leaf Scorch was the suspected disease due to the large yellow halo that bordered the scorched part of the leaf. Furthermore, the yellowing and scorch did not have any particular pattern across the leaves. This is unlike many salt issues that typically have scorching int he tip of the leaf due to accumulations of salts.
Picture 2: Symptoms of almond leaf scorch on an almond leaf. Note obvious yellow halo surrounding the necrotic leaf region.
For a true confirmation of almond leaf scorch, a diagnostic PCR assay must be completed. Leaves were taken from infected and healthy looking trees and sent to Dr. Bruce Kirkpatrick's lab in the Department of Plant Pathology at UC Davis. The diagnostic assay came back positive for the diseased trees, and negative for the healthy trees, confirming that the pathogen was present within the orchard.
Almond Leaf Scorch is caused by the xylem-limited bacterium, Xylella fastidiosa. The bacteria is very fastidious and is hard to culture in media. Artificial inoculations of almond trees made within the laboratory has shown that it can take several years for the first leaves to show symptoms after inoculation. Eventual tree death occurs through the clogging of the vascular tissues, which prevents the flow of nutrients to the branches. Leaf symptoms are caused by bacterial produced toxins that accumulate within the leaves.
The bacterium can not directly infect the almond tree by itself. It requires infested xylem feeding insects to carry and introduce the bacteria into the tree. Within almond, this is primarly limited to sharpshooters (Cicadellidae) and spittlebugs (Cercopidae), with green sharpshooters (Draeculacephala minerva) being the most commonly found in orchards. Leaf hoppers have not been shown to transmit the disease at high levels.
Picture 3: A close up of a green sharpshooter.
Xylella fastidiosa is commonly found in riparian areas due to the high diversity of plants that can serve as hosts for the bacterium. Coincidentally enough, these areas are also the same locations in which sharpshooters and spittlebugs overwinter and feed year round. Sharpshooters transfer the bacteria to the almond tree by feeding on infected ground cover and weeds and then move to feed on the tree. Upon piercing into the xylem tissue of the tree, the sharpshooter injects the bacteria into the xylem, thus infecting the tree. Tree to tree transmission of the disease has not been demonstrated and is thought not to occur. Known weed hosts include fillory, chickweed, London rocket, shepherd's purse, malva, ivy, clover, bluegrass, and stinging nettle.
Picture 4: A close up of several spittlebugs found in California.
Research by Dr. Kent Daane (UC Berkeley) has shown that X. fastidiosa is detected within weeds only between October and April, and within insects from April to July. This suggests that the ground cover and weeds are infected with almond leaf scorch in the winter months and the sharpshooters acquire the bacteria when emerging from diapause around mid-April. Tree infection is thought to occur between April and July as emerged sharpshooters can feed on the soft tissue of almonds, infecting the trees upon feeding. Sharpshooter populations within the orchard were found to be the highest during these months. Evidence suggests that orchard practices that remove the sharpshooter habitat, such as year-long weed management, may decrease the populations of shapshooters, therefore decreasing the incidence of almond leaf scorch within orchards.
Management of the disease is through removal of disease tissues. Typically, by time a strike of almond leaf scorch is seen, the infection took place 2-3 years prior. Prune out branches that are infected with almond leaf scorch by cutting back as far as possible from the last visibly affected leaf. It has been demonstrated that cuts need to be as far back as five feet from the last infected leaf. Removing too little of the branch may not remove all of the pathogen, thus allowing the bacteria to continue infecting the tree. If a high percentage of the orchard is infected, it may be in the best interest to remove the orchard. Orchard removal of older blocks may not be necessary as the progression of the disease may take longer to kill the tree than the planned orchard life.
Chemical control options are very limited for trees infected with almond leaf scorch. Tree recovery has been demonstrated with injections of the antibiotic tetracycline. This process is very expensive and time consuming and may not be worth the effort. Research also has demonstrated that it is nearly impossible to to reduce the number of vectors through applications of insecticides. There is some possibility that a "cold curing" may occur, in which bacteria titer and disease progression are reduced after cold temperatures, but this has not been clearly demonstrated.
Almond leaf scorch is found more frequently in the Peerless and Sonora varieties. Nonpareil is often affected as well. Rarely is the disease seen in Carmel or the Butte varieties. It is also thought that some varieties may "cold cure" more easily, but, again, this has not been thoroughly researched.
My recommendations for an orchard infected with almond leaf scorch is as follows:
Young Orchards (3-8 yrs): rogue out infected branches aggressively, making cuts five feet from the last infected leaf. If tree removal is necessary, replant to maintain orchard.
Old Orchards (5-7 years before planned orchard removal): It is not necessary to remove infected branches as it will take the disease longer to kill the tree than planned orchard life. Furthermore, replanting young trees may be more trouble than it is worth as they may not bear almonds before orchard removal.
Mature Orchards (8-16 years): This becomes a management decision. If the infected tree yields are low, remove the trees. Rogue branches, if it is economically possible. In the orchard described above, the decision was made to remove all of the Sonora trees and aggressively rogue out infected branches within the Nonpareil.
Publication 8106 provided by the University of California Agricultural and Natural Resource Division is available for more information.
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