Wageningen Agricultural University, Department of Phytopathology, PO Box 8025, 6700 EE Wageningen, The Netherlands

Background and objectives

During the last decade, white rust caused by the fungus Albugo candida has become an important disease in cabbage cultivation in several European countries. This disease can cause severe damage, especially in Brussels sprouts. However, little is known about the epidemiology of white rust in cabbage and methods to control the disease are limited. Multiple races of A. candida have been described and each race appears to have its own host range [1]. In this study we investigate (1) the effect of various abiotic conditions on the epidemiology of A. candida, and (2) the level of host specialization.

Materials and methods
The influence of temperature, leaf wetness and relative humidity on release and germination of spores, on infection and development of A. candida on Brussels sprouts was studied under controlled conditions. In field experiments, infection and dispersal of A. candida on Brussels sprouts was investigated from point inoculations in time. Climatic data were collected and used in the analysis. Host specialization was studied in bioassays with multiple host plants and various isolates of A. candida collected from each of these hosts. Interaction of A. candida and host plants was studied on cotyledons and true leaves.

Results and conclusions
Asexual zoospores were released between 5 and 20C with an optimum temperature of approximately 13C. Infection of Brussels sprouts occurred from 5 to 25C and latency periods ranged from 6 to 37 ;days. For the release of zoospores from sporangia and the infection of the plant only a few hours of leaf wetness were necessary. Microscopical analysis showed that at 20C within 1 ;h almost all of the zoospores were located on the plant stomata and had started to form appressoria from which the primary hyphae developed. At 20C, a period of 3 ;h of leaf wetness was sufficient to cause infection. These results indicated that the climatic conditions in The Netherlands and other west European countries are very conducive to white rust infection and disease development.

Preliminary results suggested that host specialization of A. candida is not as strict as the current classification of the races indicated. For example, an isolate from sheperds' purse (race 4) was able to infect seedlings of both Brussels sprouts and cauliflower which only can be infected by race 9 [2]. Also, several other combinations showed compatible interactions. The interaction of A. candida with mature plants will be investigated and the use of molecular techniques (e.g. AFLP analysis) to study host specialization of A. candida will be discussed.

1. Pound GS, Williams PH, 1963. Phytopathology 53, 1146-1149.
2. Hill CB, Crute IR, Sherriff C, Williams PH, 1988. Cruciferous Newsletter 13, 112-113.