Bot Gummosis (Formerly Dothiorella Gummosis)

Bot gummosis can cause branch cankers, dieback, and gummosis. Portions of trunks or branches will have dead outer bark located over a canker. The dead bark may exude gum; the cambial layer of wood underneath the bark may be brown to yellowish. The canker may spread up and down the cambium in grooves with some faint, shallow, yellowish-brown discoloration of the underlying wood.

Bot gummosis can cause rapid decline and death of a tree. Young trees are especially susceptible if the affected tissues are not removed. Often, the dead bark remains attached to the tree so tightly that it is not immediately obvious that it is dead. This dead bark has a more grayish cast than healthy bark.

Among the canker pathogens, Neoscytalidium spp. produce spores in a powdery mass that can be easily airborne and spread to tree wounds.

Unlike dry root rot of citrus, pathogen-caused bark discoloration is lighter, and infected bark may ooze dark liquid. On the surface, bot gummosis may have a grayish cast with dead bark that remains tightly attached. However, the Neoscytalidium cankers cause the outer layer of the bark to crack, peel back in later stages, and expose a mass of black powdery spores (arthrospores) that can easily spread by air.   

In the summer of 2024,  there was an epidemic of Neoscytalidium spp. on grapefruit varieties (particularly cv. Star Ruby) in the Central Valley and especially Southern California deserts (Coachella and Imperial Valleys).

Look for diseases and disease-promoting conditions regularly throughout the orchard. Where branch canker and dieback is a problem, rely primarily on sanitation and good cultural practices to control it. This strategy is based on results from studies on avocado and grapevine; however, the efficacy of those cultural practices has yet to be evaluated in citrus.

• Prune dead limbs, twigs, and cankered branches carrying spore-forming structures at least five inches below infected wood.
• Remove pruning waste from orchards before precipitation.

Pruning wounds, mechanical injury, frost, and sunburn damage have been reported as points of entry for spores in the Botryosphaeriaceae family on other hosts. In avocado and grapevine, spores are released from fruiting bodies present on branches or twigs and spread by rain splash and wind, typically between November and April.

Currently, no effective fungicides are available to prevent bot gummosis on citrus in California. However, some Trichoderma-based biofungicides were found to be effective in protecting pruning wounds on other hosts or on citrus for other diseases (e.g., Hyphoderma canker).

Neoscytalidium canker in grapefruit growing areas in Central, Coachella and Imperial Valleys

For grapefruit orchards in the Central Valley affected by canker caused by Neoscytalidium spp. follow the following additional practices:

• Avoid regular pruning during the summer because airborne spores easily infect recent pruning wounds.
• Prune out infected branches, remove them from the orchard, and burn them. Contact your county agricultural commissioner’s office for burn permit. Do not shred infected branches within the orchard. Shredding releases airborne spores that can easily cause infection in open wounds. Burning the prunings is essential to reduce inoculum levels in the orchards and the potential for additional infections.
• Apply registered pruning wound protectants immediately after pruning.
• Provide sufficient water. Grapefruit cultivars appear to be sensitive to heat. Leaf scorch observed after heat waves weakens the trees.