Fungal Canker Diseases Affect Productivity of Sweet Cherry Orchards
Mohamed T. Nouri, Orchard Systems Farm Advisor
Florent P. Trouillas, Plant Pathologist,UC Kearney Research and Extension Center
Canker diseases caused by plant-pathogenic fungi are some of the main factors limiting the productivity and longevity of sweet cherry trees in California. These diseases affect the wood, killing branches, scaffolds, and trunks of cherry trees, causing extensive damage and significantly reducing annual yields. Canker diseases are commonly found in mature cherry orchards throughout San Joaquin County, where the inoculum of these various diseases is present. Main fungal canker diseases include Eutypa dieback, Calosphaeria canker and Cytospora canker, and these must be distinguished from Bacterial canker caused by the bacterium pathogen Pseudomonas syringae.
What is a canker?
A canker in woody plants normally refers to a localized dead area in the bark and cambium of stems, branches or twigs caused by fungi, bacteria, or abiotic agents.
Most plant pathogens are unable to penetrate bark directly but will quickly colonize wounded tissue. Most canker diseases are caused by fungi, which grow within the tree’s bark and vascular tissues, killing the wood and bark of trees. In sweet cherry, most cankers are wood cankers, often initiating from the pith and heartwood, and then spreading into the sapwood.
What are the main fungal canker diseases of sweet cherry in California, and what are their symptoms?
Cytospora branch canker/dieback: also known as Leucostoma canker. The fungus attacks the woody parts of the trees through bark injuries and pruning cuts.
Symptoms of Cytospora branch canker/dieback (Fig. 1A) include longitudinal cankers in branches and scaffolds associated with the exudation of gum at the point of infection. Cytospora fungi have traditionally been thought to be of relatively minor importance on sweet cherry; however, their prevalence in cankers suggests that this group constitutes virulent pathogens. Overall, this group of pathogens has become of increasing concern in recent years in many fruit and nut crops.
Calosphaeria canker: caused by the fungus Calosphaeria pulchella. Cankers are generally initiated around the pith and progressively invade the xylem, cambium, phloem, and cortical tissues (Fig. 1B). External symptoms of Calosphaeria canker are habitually less visible during the early stages of infection, particularly in large diameter scaffold branches. In the late stages of infection, cankers are noticeable and are usually associated with symptoms of branch dieback and leaf desiccation. Perithecia (fruiting bodies) of C. pulchella are produced beneath the periderm of infected scaffold branches and trunks (Fig. 1C). These are also important signs of the disease, easily detectable, and they can allow for field diagnosis of Calosphaeria canker.
Eutypa dieback: caused by the fungus Eutypa lata is also a common disease of apricot and grapevine. Infection generally occurs during rainy weather happening at the time of pruning. Symptoms in sweet cherry include wood cankers, wilting and death of branches. Limb dieback may occur several months or years after infection (Fig. 1D). Cankers mostly originate from pruning wounds on limbs or trunks, and the bark becomes dark with amber-colored gumming. Eutypa lata spreads to new pruning wounds by wind-driven ascospores released during fall and winter rains.
Management guidelines:
Symptoms of canker diseases in sweet cherry, A. Cytospora branch canker/dieback, B. Calosphaeria branch canker, C. Circinate groups of C. pulchella perithecia beneath the periderm, D. Eutypa branch canker/dieback.
Canker diseases are best managed using a preventive as well as avoidance approach. Most importantly, pruning should be performed to avoid rain and when dry weather is predicted for at least two weeks. Pruning wounds serve as the main entry point for canker pathogens, and these must be protected following pruning of trees during the wet winter months. Sprinkler irrigation that wets the tree trunks may act just like rain and should be avoided. Water from rain and sprinkler irrigation combined with wind are important factors for aerial dissemination of canker diseases. Dr. Trouillas’ laboratory recently evaluated the efficacy of different compounds to protect pruning wounds from infection by canker pathogens. Of the different fungicidal compounds tested, Topsin M and Quilt Xcel performed best against canker pathogens of sweet cherry allowing significant disease reduction. Fungicide products should be applied immediately after pruning to avoid the contamination of pruning wounds. When orchards are seriously infected with canker diseases and branch dieback occurs, remove diseased branches/limbs at least 4 to 6 inches below any sign of wood discoloration. The pruning cut should be made into healthy wood to ensure that all the disease has been removed. Incomplete canker removal wastes time and money with little to no benefit in disease management. Dead branches left in the orchard or adjacent to living trees provide inoculum for further infection and should be removed and destroyed. It is also advised to regularly disinfect pruning shears, particularly after cutting through dead wood, using common sanitizers or a flaming torch.