How to Manage Pests

UC Pest Management Guidelines

COMMENTS ON THE DISEASE
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 develop 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. These infections kill and discolor the wood deeper than gummosis itself.

MANAGEMENT
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.

Cultural Control
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, you can halt disease spread by removing the dark, diseased bark and a buffer strip of healthy, light brown to greenish bark around the margins of the infection. 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. Recheck frequently for a few months and repeat if necessary.

Organically Acceptable Methods
Cultural controls and copper treatments are acceptable for use on organically certified citrus.

Monitoring and Treatment Decisions
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 is 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 infested 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.

Common name			P.H.I.+
(trade name)		Amount to Use	(days)
	PREPLANT FUMIGATION
A.	METAM SODIUM*	100 gal/acre
		...or...
	(Vapam)	16 fl oz/tree (8 ft diameter canopy)
	COMMENTS: Apply with 6-12 inches of water. Do not plant for at least 45 days.
B.	METHYL BROMIDE*	350-450 lb/acre
		...or...
		1 lb/100 sq ft
	COMMENTS: Use low rate on sandy loam and high rate on heavier soils with a high clay content. Inject 7-8 inches, 12-18 inches apart, and tarp immediately. Do not plant for at least 1 month.
C.	CHLOROPICRIN*	400-500 lb/acre
		...or...
		16 oz/tree (8 ft diameter canopy)
	COMMENTS: Use low rate on sandy loam and high rate on heavier soils or high clay. Inject 7-9 inches, 12-18 inches apart, and tarp immediately. Do not plant for at least 3 months.
	POSTPLANT
A.	COPPER#	Label rates
	COMMENTS: Use neutral, spray-dried, 1 package copper spray materials stirred into water to make consistency of house paint. Apply as paint or spray on trunk and crown right after excision of diseased bark; treat excised area and lower trunk. Can also be used as a protectant on trees where risk of gummosis is high.
B.	FOSETYL-AL	2.5-5 lb/5 gal water	30
	(Aliette) 80WDG
	COMMENTS: Spray or paint on trunk when disease occurs or conditions favor disease development. Use higher rate if trunk lesions are present. Thoroughly wet the lesion. If no lesion is present, wet the trunk from the ground up to a height of 2 feet. Do not exceed 4 applications of fosetyl-al/year.
C.	MEFENOXAM	1 qt/3 gal water
	(Ridomil Gold) EC
	COMMENTS: Use when disease occurs. Spray the surface of trunks to cover lesions thoroughly. Can be applied up to 3 times/year, but do not make soil and trunk applications of mefenoxam to the same tree during the same cropping season and do not apply more than 1.5 gal/treated acre/year.

+	Preharvest interval. Do not apply within this many days of harvest.
*	Permit required from county agricultural commissioner for purchase or use.
#	Acceptable for use on organically grown produce.

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Diseases
H. D. Ohr, Plant Pathology, UC Riverside
J. A. Menge, Plant Pathology, UC Riverside

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