Characterisation of Botrytis species associated with neck rot of onion in Australia
Neck rot of onion has commonly been attributed to two fungal pathogens, Botrytis aclada (syn. B. allii) and B. byssoidea. However, in other countries, a recently developed PCR technique using sequence characterised amplified region (SCAR) primers (BA2f/BA1r) followed by ApoI digestion of the amplification product, has shown the presence of two groups within B. aclada, B. aclada AI and B. aclada AII syn. B. allii. This method was used to characterise Botrytis spp. of the B. aclada/B. allii complex associated with onion seed and bulbs in Australia. A total of 24 Botrytis isolates, including two from seed and ten from bulbs (Tasmania), five from seed (South Australia), and one from seed and six from a culture collection originally isolated from five bulbs and one stipe (New South Wales), were characterised. Of the 24 isolates, one was identified as B. cinerea, one as B. aclada AI, and 22 as B. aclada All syn. B. allii.
PCR Detection and RFLP Differentiation of Botrytis Species Associated with Neck Rot of Onion.
Botrytis aclada and other Botrytis spp. can cause neck rot on onions, a storage disease that normally is very difficult to detect at harvest using traditional isolation techniques. Sequence characterized amplified region primers (BA2f/BA1r) were designed based on a previously cloned and amplified DNA fragment for direct amplification of isolates of Botrytis spp. associated with neck rot of onions. Digestion of the polymerase chain reaction (PCR) amplification product with the restriction enzyme ApoI makes it possible to distinguish the five groups: Botrytis aclada types AI and AII (B. allii); B. byssoidea; B. squamosa; and B. cinerea. The detection limit was 1 to 10 pg of pure fungal DNA. It was possible to detect B. aclada with the PCR method in artificially inoculated onion bulb tissue and in mature onion leaves showing no symptoms of the disease. The availability of a sensitive and specific PCR detection and identification method for Botrytis onion neck rot pathogens should facilitate ecological studies of this group of onion pathogens.
Evaluation and Control of Collapsible Soils
Some compacted soils wetted under certain conditions of compaction and loading may exhibit additional settlement (collapse). To investigate the effect of the soil type, compaction water content, initial dry unit weight and applied pressure at wetting on collapse potential, 138 single‐oedometer tests were conducted on eight different soils. The results indicate that well‐graded soils tend to collapse more than poorly graded ones under similar conditions. Furthermore, collapse potential decreases with an increase in (1) The difference between the sand and clay percentages; (2) compaction water content; and (3) initial dry unit weight, while it increases with pressure at wetting. A multiple regression analysis was performed on the laboratory results to provide a collapse‐predictive model that was substantiated by data reported by several researchers. A chart is presented to estimate (1) The collapse potential of soils under various conditions; and (2) the critical water content beyond which no collapse is ex...