Seasonal pattern of maturation ofVenturia inaequalisascospores in New Zealand

Abstract

Overwintered apple leaves infected by Venturia inaequalis (Cke.) Wint. were held outdoors and tested repeatedly during spring for production of ascospores. In each test leaves were placed in a wind tunnel and kept wet for 3 h to stimulate ascospore release. The ascospores were trapped with an impactor. A correction factor based on a time and temperature function was used to compensate for losses of ascospores incurred by rain between tests, when the leaves were outdoors. Tests were made during 1966-69 in Auckland ami 1966-67 in Hawke's Bay and Nelson. Some mature ascospores were always present before apple buds were at green tip stage. About 50% of ascospores had matured by full bloom in Auckland and by petal fall in Hawke's Bay and Nelson. Some ascospores did not mature until December, but generally about 95% had matured by closed calyx stage (late October-early November) in Auckland and Hawke's Bay and by mid November in Nelson. In years which had a cool spring, however, 95% maturation was not reached until 2 or 3 weeks after closed calyx..

Temperatures under glass slides used in monitoring the seasonal release of apple black spot ascospores

“…The reduced rate under the glass slides during the highest daytime temperatures appeared to be offset by the increased rate from higher under-glass temperatures in the sub-optimum to optimum range during other times of day. This study has considered the effect of temperature on ascospore release, but moisture also affects spore release (Brook 1976). It is possible that glass slides modify the moisture in the leaf litter underneath, with consequences not addressed here.…”

“…Since the rate of release of ascospores depends on temperature (Brook 1976;Gadoury and McHardy 1982;James and Sutton 1982), and since temperatures under the glass slides used for ascospore monitoring could be expected to exceed surrounding temperatures under sunny conditions, the question arises whether the glass slide method predicts natural ascospore release accurately. The purpose of this study was to compare temperatures under glass microscope slides with adjacent surface temperatures, and, using information on the temperature dependence of ascospore maturation, determine whether ascospore release estimated by the glass slide method was likely to occur earlier than in the surrounding leaf litter.…”

Influence of Air Temperature on the Release of Ascospores of Venturia inaequalis

“…In a similar work, Brook (1976) developed a linear regression equation based on temperature and hours of leaf wetness using spore trappings at 10–22°C. There is a very close relationship between the rates calculated by Brook's equation and for the temperature regimes between 10 and 18°C; at 20°C the former equation accelerates spore discharge by 1 h, while at temperatures below 10°C Brook's equation produces overestimates.…”

“…It is known that this rate is reduced at temperatures below 10°C (Hirst and Stedman, 1962; MacHardy and Gadoury, 1986; Gadoury et al, 1994), and that this reduction is high when the temperature approaches 0°C (MacHardy and Gadoury, 1986; Gadoury et al, 1994; Stensvand et al, 1997). Seem et al (1979) found little or no effect on the rate of release above 10°C, while Brook (1976) showed that increasing temperature had a significant effect.…”

Incidence ofVenturia inaequalison apple fruit during the second half of the season under different fungicide and weather regimes

“…Once high levels of disease are established, frequent applications of fungicide may be required to prevent further losses caused by secondary inoculum. The release of ascospores which initiate primary infections is usually completed by early December when the supply of overwintering ascospore inoculum is exhausted (Brook 1976) although later release can occur in dry seasons. Secondary infections by conidia released from established primary lesions can result in progression of the epiphytotic through the season.…”