Phytophthora Gummosis

An early symptom of Phytophthora gummosis is sap oozing from small cracks in the infected bark, giving the tree a bleeding appearance. The gumming may be washed off during heavy rain. The bark stays firm, dries, and eventually cracks and sloughs off. Lesions spread around the circumference of the trunk, slowly girdling the tree. Decline may occur rapidly within a year, especially under conditions favorable for disease development, or may occur over several years.

Phytophthora fungi are present in almost all citrus orchards. Under moist conditions, the fungi produce large numbers of motile zoospores, which are splashed onto the tree trunks. The Phytophthora species causing gummosis develops rapidly under moist, cool conditions. Hot summer weather slows disease spread and helps drying and healing of the lesions.

Secondary infections often occur through lesions created by Phytophthora.

Management of Phytophthora gummosis focuses on preventing conditions favorable for infection and disease development. All scion cultivars are susceptible to infection under the right environmental conditions.

Plant trees on a berm or high enough so that the first lateral roots are just covered with soil. Correcting any soil or water problems is essential for a recovery. In addition to improving the growing conditions, the following can halt disease spread

1. Remove the dark, diseased bark and a buffer strip of healthy, light brown to greenish bark around the margins of the infection.
2. Allow the exposed area to dry out. You can also scrape the diseased bark lightly to find the perimeter of the lesion and then use a propane torch to burn the lesion and a margin of 1 inch (2.5 cm) around it.
3. Recheck frequently for a few months and repeat if necessary.

Use cultural controls and copper sprays on organically certified citrus.

Late stages of Phytophthora gummosis are distinct, but early symptoms are often difficult to recognize. Yet early detection and prompt management actions are essential for saving a tree. If 50% or more of a trunk or crown region on a mature tree is girdled, it may be more economical to replace the tree than to try to control the infection.

When establishing a new orchard, carefully check the lower trunk and rootstock of new trees for any symptoms of gummosis before you plant. When trees are wrapped in burlap, open and inspect a representative sample (at least 10% of the trees). When planting or replanting in soil infected with Phytophthora, or when a susceptible rootstock has to be used, fumigation may be helpful.

Inspect your orchard several times a year for disease symptoms. Look for signs of gumming on the lower trunk and crown, and for soil buildup around the crown; do not allow bud unions to get buried. Wrappers on young trees should be lifted or removed for inspection. When you detect gum lesions, check soil and drainage conditions.

Systemic fungicides can control Phytophthora gummosis and copper sprays can be used to protect against infection.