Symptoms and Signs
Woody tissue is mainly affected. The pathogen enters through wounds or tissue infected by other pathogens, such as olive knot galls, and can cause blights, cankers, and fruit rots.
Once the fungus has entered, it invades the branch, forming an elliptical canker that can girdle and subsequently kill small shoots and branches. The presence of B. dothidea can accelerate other diseases.
Vegetative and flowering buds that were killed during the previous fall or winter do not emerge in spring. In mid-spring (end of May to June) buds that were partially infected the previous season produce fruit clusters and shoots that develop blight from the fungus in buds. The rachises of these blighted clusters turn black, as do the shoots.
When temperatures increase in May through July, the fungus moves into shoots of the previous year, causing blighting of fully developed clusters. These blighted shoots, leaves, and clusters turn brown.
Secondary infections of clusters originate where the rachises branch; they start as small black lesions that later coalesce and cause fruit blight. Secondary infections of fruit start as round, black, pin-sized lesions, some of which will expand and decay the hulls. In late August through September, infected fruit are covered with pycnidia (black flasklike structures containing the fungus spores of the Fusicoccum sp.) and obtain a silvery color, in contrast to the noninvaded blighted fruit, which are brown.
Comments on the Disease
Sources of inoculum for this disease are rachises, shoots, and petioles killed by B. dothidea during the previous growing season that remain on the trees. Cankers can also provide inoculum for as long as 6 years.
Inoculum of the fungus can also originate from other sources, such as neighboring perennial crops and native plants, because the pathogen is capable of infecting a wide range of plants. Spores from these sources, which are released and spread during or shortly after rains, or triggered by sprinkler irrigation, cause primary infections on the vegetative and flowering buds.
Pruning injuries are the primary point of infection, but other causes of cankers and wood necrosis such as sunburn, mechanical injury, and galls caused by the olive knot bacterium Pseudomonas syringae pv. savastanoi, can also lead to infection by B. dothidea. The pathogen is also spread by birds and hemipteran insects.
B. dothidea can cause latent infections on buds, leaves, and fruit. The optimum temperature range where disease symptoms develop is between 80° and 86°F. The disease can become very severe during late spring to summer when temperatures and relative humidity in olive orchards are high.
Because cankers caused by the genus Botryosphaeria can colonize the vascular tissue very fast, it is critical to manage the disease properly.
Botryosphaeria blight is extremely difficult to control, especially if allowed to increase over several years. Currently there is no information on chemical control of this disease in olive.
The best approach to reduce both the disease level and production of inoculum within olive orchards is a combination of proper orchard sanitation, pruning, and irrigation management.
- Any debris originating from the tree should be removed from the orchard as soon as possible.
- Do not cut out branches with suspected frost damage until spring to make sure the wood is in fact dead. Pruning until June and July seems to work best. When disease incidence is low, pruning the blighted shoots and panicles during the summer can help reduce or eliminate this disease for a few years.
- Prune 1 to 2 inches into the healthy tissue.
- If there are cankers present, they should be removed by cutting 10 inches below the lower margin of the canker.
- If heavy pruning is anticipated, apply additional nitrogen.
- Additionally, trees may need to be whitewashed to prevent sunburn of newly exposed bark.
- Because spore dispersal primarily occurs when humidity is high, limit work that may require contact with trees to dry weather periods.
- Lower the sprinklers so that water does not reach the tree canopy or areas where fruiting structures of Botryosphaeria may be present. Shortening the duration of irrigation from 48 to 24 hours may also be beneficial. Irrigating only during the daytime for 12 hours in 2 consecutive days reduces the disease significantly.