Management and biocontrol of Phellinus heart-rot in California prunes

California prunes occupy 18,000 ha planted almost entirely of a single cultivar. In prune orchards, wood decay diseases have long been considered problems of older trees. However, prune growers throughout the California's Sacramento Valley report increased incidence, severity and earlier onset of decay-related limb failure and tree decline. Phellinus tuberculosus was identified as the primary cause of decay-related limb failure with infections associated with pruning wounds. More than 35 orchards were surveyed of which 86% had evidence of Phellinus infestation with symptoms emerging in some orchards as young as 7 years old. Early onset of this disease means that management strategies must focus on prevention rather than eradication. Preliminary testing has begun on several potential control methods. The sole reliance on the cultivar 'French Improved' may have allowed for a degree of specialization by P. tuberculosus in California, necessitating introduction of more prune cultivars. However, no trends were evident in P. tuberculosus decay rates among wood blocks of selected prune cultivars and trials in living trees are ongoing. Trichoderma spp. were observed on P. tuberculosus fruiting bodies in orchards and are routinely isolated from decayed prune wood. Trichoderma is considered a mycoparasite and is considered an effective biocontrol of diseases in other crops. Trichoderma isolates collected from orchards reduced P. tuberculosus growth in culture, and several commercially available products reduced P. tuberculosus decay in wood blocks when applied prior to P. tuberculosus inoculation. Field testing of Trichoderma containing products as a pruning wound protectant are underway. Conventional maintenance pruning involves removal of interior limbs to maintain a vase-like structure, leading to P. tuberculosus infection and limb breakage near the crotch of the tree. Less incidence and severity of disease was observed in orchards that were mechanically hedged, suggesting alterations to pruning practices may lessen impacts of P. tuberculosus infection.