First, the symptoms
Young tree trunks are particularly susceptible to infection, typically within 10 to 15 centimeters from the ground. The affected areas appear pale brown to grayish in color, slightly sunken, with the bark breaking and exuding yellow-brown mucous. The xylem becomes discolored, turning brown, and shows visible circular necrosis lines across the base of the trunk. In the later stages, small black spots can be seen under the broken bark—these are the fruiting bodies of the pathogen. Leaves may also be affected, showing gray-brown, round or irregularly shaped lesions. Affected leaves turn yellow, their veins become transparent, and the leaves may wilt. In severe cases, the entire plant can dry out and rot.
Second, the pathogen
The causative agent of yellow gum disease is known as Phomopsis wampi, classified under the subgenus of Deuteromycotina. The fruiting bodies and their spore-producing structures are found beneath the bark, appearing dark brown. Within the fruiting bodies, two types of conidia are produced: Type A conidia are colorless, single-celled, straight or slightly curved, and oblong in shape; Type B conidia are also colorless but have a hooked structure. This pathogen only affects yellow gum trees and does not cause disease in citrus or mango plants.
Third, the infection cycle and disease conditions
The yellow gum pathogen overwinters beneath the damaged bark. As spring arrives, the pathogens in the infected areas produce a large number of conidia, which are spread by rain or wind. The pathogen's virulence is relatively low, and it usually infects weakened trees or those with wounds. It enters through these openings and causes disease. Excessive rainfall in spring creates favorable conditions for disease development. The peak period for this disease occurs annually between April and May. If left unchecked, the gummy disease can cause significant damage to the trees.
Fourth, control methods
To manage the disease effectively, it is essential to combine good agricultural practices with targeted treatments. After harvest, proper care should be taken to maintain tree health by tilling the soil and applying organic fertilizers. Regular inspections during spring are crucial. If any signs of infection are detected, such as oozing sap, immediate action should be taken. The treatment involves using a knife to remove the infected tissue, followed by applying a mixture like 1:1:10 Bordeaux paste, or a diluted solution of 50% carbendazim wettable powder (100–200 times), or 50% Topozin wettable powder (100 times). These methods have proven effective in controlling the disease and promoting recovery.
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