Phytophthora gummosis of citrus—Phytophthora spp.
Trunk disease from infection by Phytophthora species is called Phytophthora gummosis. Infection by at least Phytophthora citrophthora and P. parasitica can cause bark on the lower trunk of citrus trees to crack and ooze dark liquid. Various Phytophthora species can also cause brown rot of citrus fruit and Phytophthora root and crown rot and these maladies can be present along with Phytophthora gummosis.
Identification
Bleeding from small cracks in bark on the lower trunk is commonly the first external symptom of Phytophthora gummosis. Oozing bark initially is firm, but later dries, cracks, and sloughs off. Lesions enlarge around the circumference of the trunk, slowly girdling the tree unless effectively managed. Tree decline and death from trunk girdling can occur rapidly within a year, especially under conditions favorable for disease development, or can occur over several years.
When bark is cut off early in disease development, it can be observed that the Phytophthora infection does not discolor wood. But secondary pathogens commonly infect through the Phytophthora lesions and these later-arriving pathogens can kill and discolor wood.
Bark gumming due to Phytophthora species may be obvious only seasonally. Gum can be washed off during heavy rain or where trunks are directly wetted by irrigation sprinklers.
Lookalikes. Gumming alone is not definitive for diagnosis because other diseases sometimes cause ooze on the trunk of citrus. These include bot gummosis, formerly called Dothiorella gummosis. Bot gummosis is caused by various Botryosphaeriaceae family pathogens including Diplodia, Dothiorella, Lasiodiplodia, and Neofusicoccum species and Neoscytalidium dimidiatum. Hyphoderma gummosis, caused by Hyphodontia (=Hyphoderma) sambuci, also produces trunk bleeding. However in addition to oozing and cankers on the lower trunk, bot gummosis and Hyphoderma gummosis commonly cause cracked and bleeding bark on limbs. Phytophthora gummosis causes disease only low on the main trunk.
Life cycle
Phytophthora are present many places. Under moist conditions, these oomycete (water mold) pathogens produce large numbers of motile zoospores, which are splashed onto the tree trunks and cause infection. The Phytophthora species causing gummosis develop rapidly under moist, cool conditions. Hot summer weather slows disease spread and helps to dry and heal the lesions.
All scion cultivars are susceptible under environmental conditions that promote disease. If the rootstock is susceptible to Phytophthora infection, the disease can spread into the crown and woody roots (sometimes called foot rot). These same pathogens also cause brown rot of citrus fruit and Phytophthora root and crown rot.
Solutions
Obtain disease-free planting stock from a reputable commercial supplier. Consider planting rootstocks most tolerant of Phytophthora diseases, such as Alemow, citrange, sour orange, swingle citrumelo, and trifoliate orange.
Provide good soil drainage. Plant citrus on a raised mound of soil and keep the graft union above the level of mulch and soil. Use drip irrigation instead sprinklers. If using sprinklers, direct or shield them so water does not directly wet the root crown and trunk. Avoid overirrigating and do not cause prolonged soggy soil (waterlogging). Use sophisticated methods to estimate the amount and frequency of needed irrigation, such as according to local evapotranspiration (ET) rates or the use of soil moisture monitoring devices such as tensiometers.
Inspect trees at least several times a year and look for symptoms and signs of disease and other problems. Look for signs of gumming on the lower trunk and crown and for conditions that promote disease, such as a covered root crown and soggy soil.
Where a canker of Phytophthora gummosis is found, you can cut off or scrape away the diseased bark. Create a border or buffer strip of healthy light brown to greenish bark around margins. Allow the area to dry and the growth of the canker may stop. After cutting out the diseased bark the wound can be sprayed promptly with Bordeaux mixture or another copper fungicide to reduce the likelihood of reinfection. Alternatively, fosetyl-al can be sprayed on the lower trunk. If infection recurs, these procedures can be repeated.
Adapted from Integrated Pest Management for Citrus and Pest Management Guidelines: Citrus, University of California Statewide Integrated Pest Management Program (UC IPM), and Phytophthora Disease of California Citrus, UC Agriculture and Natural Resources. |
Gum oozing from small cracks in trunk bark is commonly the first external symptom of Phytophthora gummosis.
Bleeding from the lower trunk due to Phytophthora gummosis.
Cracked, dry bark on the basal trunk due to Phytophthora gummosis.
To stop the spread of Phytophthora gummosis, cut off or scrape away diseased bark and create a border or buffer strip of healthy light brown to greenish bark.
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