1HortResearch, Mt Albert Research Centre, Private Bag 92169, Auckland, New Zealand; 2University of Auckland, Private Bag 92019, Auckland, New Zealand

Background and objectives

Venturia inaequalis causes apple scab, the most commercially important fungal disease of apple worldwide. It causes lesions on leaves and fruit, reducing both quality and quantity of the harvest. Currently, none of the commercial apple cultivars used in New Zealand (NZ) are resistant to the disease and control in the orchard relies on a rigorous fungicide regime. Breeding resistance to V. inaequalis into commercial apple cultivars is a major focus of apple breeding programmes worldwide. Successful deployment of resistance requires an in-depth understanding of the variation in the pathogen population. Seven races of V. inaequalis have been identified. Little is known about the variation in the NZ V. inaequalis population, but only one race (race 1) has been detected. A preliminary study using RAPD analysis showed an extremely high degree of variation in the fungus in NZ; no two isolates had the same RAPD banding pattern. This was not entirely unexpected since V. inaequalis reproduces sexually every growing season. We have extended the survey of NZ population to include more growing regions, closely related isolates from apple and isolates from crab apples. These were compared with isolates from Northern Hemisphere populations. In the present study, we used both RAPDs and ribosomal DNA (rDNA) sequence analyses to characterize genetic variation at the molecular level.

Materials and methods

Monoconidial V. inaequalis isolates were collected from individual lesions on infected leaves and fruit. Host material included different apple and crab apple cultivars from the Auckland region and from various apple-growing regions in NZ (to maximum distance of approximately 1000 km). The isolates from crab apple were included to determine whether ornamental trees could host a different subpopulation of V. inaequalis. Crab apple species are also the major source of resistance genes currently being used in apple breeding programmes.

Results and conclusions
RAPD analysis revealed a high degree of variation in the NZ V. inaequalis population. Only isolates collected from the same leaf or lesion had identical banding patterns. Some isolates from different lesions on the same leaf produced different RAPD patterns. All isolates shared common major bands, some of which have also been reported by other researchers [1] for V. inaequalis from a Swiss orchard. There appears to be no correlation between RAPD banding patterns and the origin of the isolates (crab apple or apple cultivar, or geographical location). Ribosomal DNA analysis showed five different sequence classes within the NZ population; however, these classes did not correlate with any region or cultivar. All isolates contained an intron of 387 bp, which differs from a Swiss study [2] where an additional class of V. inaequalis lacking the intron was reported. Phylogenetic trees were generated from the RAPD and the rDNA data. Bootstrap analyses showed that there was no significant structure to any of the trees generated for the NZ collection. Analysis of intron sequences for V. inaequalis, obtained from the GenBank grouped the European isolates with those from the NZ isolates. The high degree of variation in the NZ V. inaequalis population may indicate that panmixis is occurring. Alternatively, the variation could be due to the population having arisen from many separate introductions. Unlike the situation in the Northern Hemisphere, NZ has only race one and this race can be controlled with existing Malus resistance genes, eg. Vf and Vm. If panmixis is occurring in the NZ population this will have implications for the deployment of resistance genes in this country.

1. Sierotzki H, Eggenschwiler M, McDermott J, Gessler C, 1994. Norwegian Journal of Agricultural Science 17, 83-93. 2. Tenzer I, Gessler C, 1997. European Journal of Plant Pathology 103, 565-571