Bacterial Canker of Kiwifruit: Response to a Threat

Bacterial canker of kiwifruit caused by a virulent strain of Pseudomonas syringae pv. actinidiae (Psa) is having a major effect on kiwifruit industries throughout the world. Already many orchards have been destroyed, especially those of the newer kiwifruit cultivars with gold fruit flesh, and continued cultivation of some of these has become commercially non-viable. The disease can also affect 'Hayward', if to a lesser extent, and we do not yet know the longer-term prospects for this cultivar. Eradication of the disease seems unrealistic so we have to learn how to live with it, to prevent or minimize its spread and to help kiwifruit vines to survive. Much of the scientific effort so far has focused on the bacterium, its genetics and biology and the possible origins of the virulent strains. The epidemiology of the disease has also been studied, how the bacteria penetrate into the vine and chemical methods that might control development of the infection. The results obtained with different chemicals have been often contradictory. Even so, many orchards are sprayed with several active ingredients - no longer can kiwifruit be considered as needing few protective sprays. More rigorous orchard hygiene and restrictions on the movement of nursery plants may help to limit the spread of the disease. Cultural practices have been tested to determine what relationship they might have with severity of the symptoms but in at least some cases the effects on yield and on fruit quality have been ignored or not properly assessed. Breeding or selection of resistant or tolerant cultivars seems the ideal solution. Priorities in breeding programmes must change and selection for disease resistance has obviously become more important. However, a disease-resistant cultivar with poor fruit quality is not a solution and breeding programmes are likely to take time.