Understanding disease vectors and recognizing disease symptoms can allow for timely management decisions in almond production.
Mohammad Yaghmour, University of California Cooperative Extension (UCCE) orchard systems advisor for Kern and Kings counties, explained differences and similarities among vector-transmitted bacterial and viral disease of almond trees and how the diseases are transmitted at the South Valley Nut and Citrus Conference.
A typical disease triangle includes host, pathogen and environment, Yaghmour said, but vectors can also play a part in spread of disease.
Almond Leaf Scorch
The bacterial disease Almond Leaf Scorch (ALS) is vectored by insects and can become a chronic problem in almond orchards, reducing yields and eventually causing tree decline and death. Yaghmour said the symptoms of ALS might not even be noticeable for several years after infection. Symptoms of this disease can be spotted in early June. Later in the growing season and close to harvest time, symptoms of ALS could be hard to recognize if the trees are suffering from other stresses. Such symptoms which include dry or scorched appearing leaf margins may be attributed to salt burn or water stress.
The distinguishing symptom for ALS is a yellow band on affected leaves between the healthy tissue and the dry tissue. This slow developing disease may first affect one branch or a portion of one scaffold near the infection site, but as the disease progresses, the entire canopy becomes affected. Trees on orchard edges are typically affected first. Yaghmour said that symptomatic leaves can be tested early in the season when symptoms first appear to make a positive ALS diagnosis.
The bacterium that causes ALS is Xylella fastidiosa, also causes Pierce’s Disease in grapevines, and alfalfa dwarf disease. It was first discovered in California in Mendocino County in the 1940s and then found in Los Angeles and Contra Costa counties.
Vectors of ALS
Insects that feed on the xylem of plants are vectors of ALS. Vectoring species include leafhoppers and spittlebugs. Sharpshooters are also vectors, but it is not believed that they can spread the disease from an infected tree to a healthy tree. The disease inoculum can be found in many common orchard weeds or in riparian plants species. The UC IPM web sites identified host plants as bluegrass, nettle, burr clover filaree, chickweed and cheeseweed. The disease has been found in all almond growing regions of the Central Valley.
Prune Back or Remove Trees
If a number of trees in an orchard have been diagnosed with ALS, Yaghmour said growers must make a decision to either prune back infected tree branches in early infection stages or remove the orchard before yields are severely affected. Infected trees that are less than ten years old should be removed. Orchards that are nearing the end of their normal life span will likely not have enough of a yield loss to justify removal right away. The most difficult management decision, Yaghmour said, is with an 11-16 year orchard. Factors in that decision include the number of infected trees and the orchard proximity to young orchards. Orchard age, yield loss due to infection, and the value of a maximally producing almond tree should be considered when deciding to remove ALS-affected trees.
Management
One of the management tools is to keep orchard floors free of weeds because infected weeds with the bacterium can be a source of inoculum. Orchards with a permanent cover cropping system are at more risk for ALS. A six-week interval where floors are bare will prevent the establishment of vectors in the orchard, Yaghmour said. Irrigated pasture and weedy alfalfa fields are the most common habitats for ALS vectors.
Almond Calico and Infectious Bud Failure
Almond calico and infectious bud failure are viral diseases. Symptoms of the disease are more common in the spring when weather has been cool. The virus that causes the disease is Prunus Necrotic Ringspot Virus (PNRV) which is transmitted by pollen mediated by insect damage to flowers. Symptoms include chlorotic spots and mottling on leaves, but they may not show clearly in years with warm, dry spring temperatures. Infectious bud failure can be moderate to severe with or without calico symptoms. Sometimes, symptoms of infectious bud failure may develop 10 years after initial infection. Lateral flowers and vegetative buds fail and few flowers set fruit.
Planting nursery trees certified PNRV-free is recommended, Yaghmour said. If Calico symptoms appear in a few young trees, removing them will eliminate the source of pollen. If Calico symptoms are widespread, replanting the entire block might be the best option.
Yellow Bud Mosaic
Another viral disease is Yellow Bud Mosaic. This disease is caused by the tomato ring spot virus and affects other stone fruit including peaches and apricots. It is spread in almond orchards by using infected budding plant material or by dagger nematodes inside the orchard. This virus is also seed-transmitted. Infected seed will grow and then become a source of the disease in the orchard if the orchard is infested with dagger nematode.
Symptoms of trees infected with yellow bud mosaic include crinkled or distorted leaves, necrotic spots may develop and then will drop off leaving a tattered appearance. Leaves may also be stunted or form small rosettes. Hulls of developing fruit have a rough or wrinkled appearance.
Yaghmour said this disease was first diagnosed in California in 1936 in almonds and peaches in Solano and Yolo counties. The Mission cultivar is most susceptible to yellow bud mosaic which can also infect most rootstocks except for Marianna 2624. Hosts for the virus include broadleaf weeds, fruit trees and grapevines.
Management of yellow bud mosaic includes limiting nematode spread into the orchard. If smaller areas are affected, removing infected trees plus trees at least two rows from a small affected area is recommended. Infected trees should be treated with an herbicide or girdled to kill roots. As many roots as possible should be removed, ground should be fallowed and fumigated before replanting. Yaghmour said that if there are several places in the orchard with symptomatic signs of the disease, removal of the entire block should be considered.
Almond Brown Line and Decline Disease
Almond brown line and decline disease affects almond cultivars grown on Marianna 2624 rootstock. This disease is associated with Peach yellow leafroll phytoplasma, which is transmitted by pear psylla. Phytoplasma is a microbe that spreads within the phloem of an infected tree. When it reaches the graft union, cells of the Marianna rootstock die, leaving a layer of brown, necrotic cells that prevent spread of the phytoplasma into the rootstock, but also blocks nutrient movement between rootstock and scion.
Trees with almond brown line and decline are stunted. Current season shoot growth is shortened or absent. The name brown line comes from the appearance of brown areas at the graft union. To make a diagnosis check the trunk circumference. This disease is most common in young trees and has been seen in Carmel, Peerless and Price scions according to the UC IPM web site.
Infected trees should be removed. Only budwood certified clean of this disease should be used for propagation.
Cecilia Parsons
Cecilia Parsons has spent the past 30 years covering agriculture in California for a variety of newspapers, magazines and organizations. During that time she has been fortunate to witness some of the important events that have shaped this diverse industry and worked hard to examine and explain these events for readers.
When Cecilia first moved to the San Joaquin Valley in 1976, her first journalism job was at a small daily newspaper where she covered “farm news.” From there she branched out to writing for a dairy magazine and a regional weekly agriculture publication.
Cecilia is part of a farming family from the rural community of Ducor where she also raises purebred sheep and is attempting to master versatility ranch horse riding.
