Pathogenicity and genetic variation of Phytophthora cactorum from silver birch and strawberry

Phytophthora cactorum strains isolated from necrotic stem lesions on Betula pendula seedlings or from Fragaria ananassa plants suffering from crown rot were pathogenic to their host plants. Only isolates from birch caused clear lesions on non-wounded bark of birch. P. cactorum isolates from birch were not detrimental to strawberry. Random Amplified Polymorphic DNA (RAPD) analysis revealed variation within P. cactorum, isolates from silver birch having different banding patterns than those from strawberry. UPGMA analysis clustered isolates from silver birch and strawberry plants into separate groups. The data show that the recent outbreak in Finland of P. cactorum in birch could not be caused by the import of strawberry plants affected by crown rot.     

Survival,distribution and genetic variability of inoculum of the strawberry red core pathogen,Phytophthora fragariae var. fragariae,in soil

An experimental field infested with Phytophthora fragariae var. fragariae (Pff) and used for strawberry red core fungicide and cultivar resistance trials until 1981 was surveyed for the presence of inoculum of the pathogen 11 and 12 years later. Alpine strawberries, highly susceptible to all races of Pff, were grown from true seed and planted as a bait crop on a 0·5 m‐spaced grid. Rapid and widespread red core infection was observed, which provided good evidence that oospores had survived in soil for this extended period. Site elevation and the distribution of red core infected plants showed a strong correlation, with a higher frequency of infected and dead plants in the lowest areas of the field. The race designation of 18 recovered isolates were determined and AFLP fingerprint patterns of some of these and their single‐spore derivatives were analysed. The isolates differed little in race type, and the majority were genetically identical at 433 AFLP loci. Races used to inoculate the site in the 1970s were recovered. The fingerprints of the single variant isolate matched that of an isolation made by Hickman in the 1950s, originally used to inoculate the site. Clearly Pff is a very stable and long‐lived pathogen able to retain its genetic integrity and lie dormant in soil for many years, ensuring its survival between epidemiologically favourable conditions which occur erratically.     

On-farm selection for quality and resistance to pest/diseases of cocoa in Sulawesi: (ii) quality and performance of selections against Phytophthora pod rot and vascular-streak dieback

The cocoa industry in Sulawesi, the main region of cocoa production in Indonesia, is threatened by destructive diseases, including vascular-streak dieback (VSD) caused by the basidiomycete Oncobasidium theobromae and stem canker and Phytophthora pod rot (PPR) or black pod, caused by Phytophthora palmivora. Using the considerable genetic diversity of cocoa on farms, host resistance was identified and tested with the participation of farmers. Forty-nine local and international cocoa selections with promising resistance characteristics (as well as susceptible controls) were side-grafted onto mature cocoa in a replicated trial with single-tree plots. Developing grafts were assessed in the dry season for severity of VSD infection, scored from 0 (no infection) to 4 (graft death). All of the 49 clones in the trial became infected with VSD in at least some replicates. Average severity varied from 0.2 to 1.6. Potential VSD-resistance was found in eight clones, including DRC 15, KA2 106 and a local Sulawesi selection, VSD2Ldg. Some of the most susceptible clones were local Sulawesi selections from areas with a history of little or no VSD. Thirty-four pod-bearing clones were evaluated over a 2-year period for yield, quality and resistance to natural infections of PPR. Cumulative PPR incidence for all clones was 22% but varied from 8.6 to 43% among clones. Clones with less than 15% PPR incidence were designated as resistant, including DRC 16 and local Sulawesi selections, Aryadi 1, Aryadi 3 and VSD1Ldg. Scavina 12 was moderately resistant in the trial with a PPR incidence of 23%. Cumulative incidences of the mirid, Helopeltis spp., determined in the same evaluation period, indicated that DRC16 was the most susceptible clone with an incidence of 52% in ripe pods and 23% in immature pods. In comparison, KKM4 showed evidence of resistance to Helopeltis spp., with incidences of 34 and 0.8% in ripe and immature pods, respectively. The impact of diseases and pests (including cocoa pod borer) on bean losses and bean quality varied between clones but generally the bean size (or bean count) was affected more than the fat content or shell content.