Epidemiology and effective control of Altenaria altenata, causal agent of dead (dormant) flower bud disease of pear

Dead flower buds are a common phenomenon in pear culture in The Netherlands, Belgium and Mediterranean countries. Disease cases are also reported from South America. The disease is characterized by a partial or complete necrosis of flower buds during tree dormancy. The disease progresses during winter and spring, eventually resulting in the death of most flowers and decay of buds at flowering. In The Netherlands the problem is mostly found in the main pear cultivar ‘Conference’, but cultivars such as ‘Doyenne du Comice’ and ‘Gieser Wildeman’ are also affected. Disease incidence may be as high as 80-90%. Possible causes mentioned are abiotic stresses, incompatibility between scion and cultivar, and plant pathogens and pests. Research in recent years revealed that pear growth regulation does not prevent the occurrence of dead flower buds. Also, the bacterium Pseudomonas syringae pv. syringae (P.s.s.) was commonly regarded as the causal agent of dead flower buds in pear, although the relation between P.s.s. and dead flower buds in orchards has never been proven in The Netherlands. However, the fungus Alternaria alternata was found in diseased flower buds and also often in symptomless flower buds. A linear relationship between infection rate and dead flower bud disease incidence was found. Pathogenicity tests and Koch’s postulates were carried out. It was concluded that A. alternata is the causal agent of dead (dormant) flower bud disease. A. alternata is known to cause late blight in pistachio and several diseases in fruit crops such as moldy-core in apple and brown rot in citrus. By identifying the causal agent of dead flower bud disease, an effective control strategy could be developed. In field trials it was proven that fungicide treatments can reduce disease incidence significantly.