A newly described Ascomycete, Eutypa armeniacae Hansf. & Carter, is shown to give the symptoms associated with "gummosis" disease of Prunus armeniaca L. (common apricot) in southern Australia. Experimental evidence is produced to substantiate the hypothesis that E. armeniacae is the ascigerous stage of an imperfect Cytosporina sp, which has long been recognized as the causal organism of "gummosis", and that airborne ascospores of E. armeniacae are the only source of inoculum. Ascospores of this species are shown to be well suited to aerial dissemination over long distances.
Perithecia and ascospores of E. armeniacae may be produced on dead apricot wood for at least 5 years following maturation of the first stromata. Each winter, a stroma may produce new perithecia between the exhausted ones of the previous year. Results from a three-year quantitative study of the seasonal abundance of airborne ascospores show that, in South Australia, there is a winter period of low ascospore frequency which coincides closely with the dormant period of the apricot host. Abundant ascospore release follows rainfall exceeding 0�05 in. at other times of the year. The pattern of ascospore release, when related to natural rainfall in field tests and to moisture supply in laboratory tests, gives no evidence of diurnal periodicity.
Studies on the spread of leaf scald of barley from an infection focus showed that infected straw was the source of primary inoculum. The disease progressed mainly between adjacent plants but pockets of infection appeared in the crop at isolated positions several metres away from the source of inoculum. Sporulation occurred when free water was available and the conidia were trapped simultaneously with rainfall or irrigation. Some conidia were caught under windy but rainless conditions. Conidia were trapped at any time of day or night but few were obtained at any one time.
In the field, infection of apricot occurred when inocula of 10 or 100 ascospores of E. armeniacae were applied to wounds on the day of pruning. The proportion of wounds infected was doubled when the inoculum dose was increased from 10 to 100 ascospores. On sheltered trees, by contrast, freshly-made wounds were equally susceptible (c. 90 per cent) to infection from doses of 10 and 100 ascospores. When no rain fell for 12 days after pruning, large inocula (100, 1000 ascospores) applied on day 15 caused infection of a high pro- portion of wounds on unsheltered apricot trees. Infection of almond, peach, and prune was also induced by inoculation. Although there was extensive xylem invasion in these hosts, the fungus did not produce the symptoms of cankering and lateral collapse seen in apricot. Our results suggest that the number of ascospores naturally deposited on individual pruning wounds is normally less than 10, and that inocula of this size therefore are more appropriate than the larger doses used previously.
Results of trapping M. pinodes ascospores with a Hirst volumetric spore Trap during two successive crop seasons have demonstrated the pattern of variation of airborne inoculum in a South Australian irrigated pea field. Ascospore concentrations in the air above the field can be related to the presence of mature perithecia in residual crop debris and to the maturation of new perithecia on plants of the current crop. Wind tunnel experiments have shown that approximately 0.007 in. rainfall is the minimum for ascospore release, and that the peak liberation from mature perithecia occurred within 0 . 5 hr of wetting the straw. Results of experiments with straw burial treatments indicated that soil temperatures in the range 28–33°C, with high moisture, will cause a rapid reduction in the numbers of ascospores which may be discharged when the straw is unearthed subsequently. Examination of perithecia in straw which no longer discharged ascospores after such treatments, however, revealed that the perithecia, contained masses of dark-coloured spores quite distinct from normal ascospores.
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