Symptoms and Signs
(View photos to identify cankers on limbs and trunks caused by pathogens)
Black streak appears as an elongated dark discoloration on bark. Small cankers can develop in a direction that parallels the direction of limb or trunk growth but sometimes cankers encircle limbs or the trunk. On green shoots and young trees, lesions look like black blotches with distinct margins. Cankered bark develops shallow cracks that ooze sap, which dries as a brownish or white powder on the bark surface. This exudate is readily washed off by rain or sprinklers, and in the absence of the powder the canker can be difficult to see externally on bark. Black streak lesions can be very small or encompass the greater part of the trunk. Cankers often first appear on the lower trunk and the underside of lower limbs and then later appear higher in the tree. Scraping off bark over the canker reveals shallow reddish brown to black areas. This discoloring forms mottled areas of dead and live tissue or merges into one large necrotic area. It rarely extends into the wood and can be removed easily by inserting a knife blade under the canker and prying upwards. Because trees can die with very few lesions, the lesions appear to be a symptom of the disease and not the cause of tree death.
Comments on the Disease
Black streak develops under adverse growing conditions and is a serious disease that can kill avocado trees. The specific cause of the disease is unknown and apparently is not a viroid as was previously believed.
Many symptoms of avocado black streak are similar to those from other causes; the appearance of the cankers is the most diagnostic characteristic of this disease. Avocado black streak appears after prolonged periods of environmental or cultural stress, especially conditions of high salinity and insufficient water. An affected tree can decline gradually and may eventually die, or it may collapse and die rapidly. Conversely, with improved cultural practices trees can recover and symptoms can virtually disappear.
Avocado black streak may occur wherever Guatemalan cultivars are grown in California. All ages of trees are affected, and symptoms have been observed on trees ranging from 1 year to over 35 years old. Many groves are apparently free of the disease, and disease incidence varies considerably within affected groves. Avocado black streak symptoms typically are most severe on trees that appear to be the most stressed.
Management
Current management of avocado black streak consists of maintaining plant health with good fertilizer and irrigation practices, and preventing stress. Adequate irrigation with high quality water is believed to be especially important since this pathogen affects drought stressed trees.