Australasian Plant Pathology

The influence of spore dose and interrupted wet periods on pear scab infection was studied under controlled conditions by assessing disease development on pear seedlings. Under optimal conditions (20°C and continuous wetness), the severity of leaf scab (lesions/cm2) increased with conidial concentrations ranging from 102 to 105 conidia/mL. Pear seedlings inoculated with conidia were also exposed to an initial short wet period (3 and 5 h) and a final 24 h wet period which was interrupted with 1, 6, 8, 12, 24, 48 or 90 h of dryness under low (<70%) and high (>90%) relative humidity. The length of the dry period reduced disease severity. During interrupted wet periods under high relative humidity (>90%) at 18" and 20°C, the number of lesions per cm2 of leaf tissue decreased from 2.7 to 0.24 and 2.7 to 0.15, respectively as the dry period ranged from 1 to 90 h. In the same two experiments with similar wet/dry/wet regimes under low relative humidity (<70%), disease severity was also reduced with no scab lesions occurring if leaves were dry for more than 12 h.

During a survey of plant parasitic nematodes associated with native vegetation in Australia, two new species of Paratrichodorus were found. Paratrichodorus orrae n.sp. was collected from Eucalyptus woodland and P. queenslandensis n.sp, from tropical rainforest, both closely resembling P. grandis. Paratrichodorus lobatus and the closely related P. teres are studied and the six trichodorid species recorded in Australia are listed.

Genetic variation among 29 isolates of Fusarium oxysporum f.sp. zingiberi (Foz) collected from diseased ginger rhizome in production regions throughout Queensland was analysed using DNA amplification fingerprinting (DAF). Eight isolates of other Fusarium species and/or formae speciales were included for comparative analysis. Within the Foz isolates, three haplotypes were identified based on 17 polymorphic bands generated with five primers. Two groups showed very little genetic variation (98.6% similarity), whereas the third single isolate was quite distinct in terms of its molecular profile (77.2% similarity). Genetic similarity among the Fusarium solani, F. oxysporum f.sp. lycopersici and F. oxysporum f.sp. cubense races 1, 3 and 4 isolates compared well with the published literature. Additional keywords: Fusarium yellows, ginger, rhizome rot, Zingiber officinale.

Fruit pathogens are currently one of the main factors involved in quarantine-related market access. Unconfirmed South African records pertaining to the presence of the causative agent of European canker of apples, Neonectria ditissima, have been the cause of many disputes regarding its status as a quarantine pest in this country. To clarify this issue, the identity of fungal isolates that were presumed to be N. ditissima, were verified by making use of species-specific PCR primers designed for N. ditissima. The isolates were obtained from symptomatic apple trees in the major apple-producing areas in South Africa. These data were supplemented by a morphological reexamination of all six previous herbarium specimens presumed to be N. ditissima. The results revealed that none of the samples tested positive for N. ditissima, nor were any of the six herbarium specimens representative of Neonectria species. This study represents the first attempt to resolve the status of N. ditissima in South Africa by means of a molecular approach, and our findings support the current regulated status of N. ditissima in South Africa.

The field symptoms of Eradu patch are distinct, stunted patches in narrow-leaf lupin and ill-thrift patches in barley. The pathogen, a thin, binucleate Rhizoctonia (TBR), is difficult to isolate with standard methods. To develop an assay specific to TBR, the ribosomal RNA ITS region was amplified and a 610 bp section including the conserved 5.8S region was sequenced. This sequence did not match any published sequence. Primers specific to TBR were constructed. The specificity of the assay was confirmed using 104 isolates of TBR obtained from many sites in Western Australia. Tests against R. solani AG 8 and other fungi commonly isolated from lupin and barley roots were negative. TBR was detected in all roots from artificially infected lupin and barley, but detection of TBR in root samples collected from field patches in 1997 was low (0–15%). New methods for extracting the TBR DNA, including the use of soil immersion plates, enabled detection of TBR (80–100%) in field samples collected in 1999. The PCR assay developed is a reliable technique for the detection of TBR in field samples and will provide an effective tool for diagnostics and the conduct of field surveys.

Seed dressing with Baytan (a.i. triadimenol) completely controlled powdery mildew for more than 60 days and reduced disease levels for the rest of the season, increasing barley yield by 11% at Esperance. In 16 field experiments in southern Western Australia representing 32 agro-climatic environments, Baytan increased yield by an average 7%; greater percentage yield increases occurred in high yielding crops with high disease in the control plots.