Symptoms and Signs
The wood decay fungus causes cankers on branches and trunks of lemon trees and is capable of invading the cambial tissue. Symptoms include
- extensive gummosis along longitudinal cracks that develop around infected tissue.
- severe wilting and dieback of the tree canopy.
- eventual tree death.
- crusty layers of pink- to cream-colored fruiting bodies of the pathogen on the surface of diseased branches and the tree trunk under favorable environmental conditions.
Comments on the Disease
The fungus has a wide host range, including many tree species, but on citrus it has been reported only from lemon. The disease occurs in the Central Valley and coastal lemon production areas of California. It can be devastating to groves in growing areas prone to environmental stress, but its occurrence is localized based on cultural practices and environmental conditions. Basidiospores are wind disseminated and colonize exposed wood such as pruning wounds or injuries caused by severe environmental conditions (e.g., frost damage). H. sambuci cannot invade the tree through intact bark. The yearly pruning practices on coastal lemons is probably the main reason for the occurrence of the disease in this region. Older, less vigorous trees are the most affected.
Management
The main management strategies are prevention of injuries and large wood-exposing wounds and maintaining healthy and vigorously growing trees.
Cultural Control
Proper pruning practices include avoiding stub and bench cuts that prevent rapid wound healing. Pruning should be delayed as long as possible in the spring when no more rainfall is expected, but it should not be delayed until summer to avoid sunburn injury. Sanitation measures include the removal of infected plant material from the grove and burning when regulations allow.
Treatment Decisions
There are no curative treatments for this disease. To protect wounds from the pathogen Hyphodontia sambuci, apply the biocontrol agent Trichoderma harzianum within 24 hours after pruning, hedging, or topping.
| Common name | Amount to use | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| FIELD-WOUND TREATMENT | ||||
| A. | TRICHODERMA HARZIANUM | |||
| (PlantShield)# | 16–32 oz/acre | 4 | 0 | |
| MODE OF ACTION: Various; biological. | ||||
| COMMENTS: Biological control treatment for protecting wood-exposing injuries that occur to branches and trunks. Applications should be made immediately after pruning or other injuries. The biocontrol agent colonizes the wood and excludes the pathogen from entry. | ||||
| # | Acceptable for use on organically grown produce. |
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |