EFFECTS OF TEMPERATURE, VAPOR-PRESSURE DEFICIT AND RADIATION ON INFECTIVITY OF CONIDIA OF BOTRYTIS-CINEREA AND ON SUSCEPTIBILITY OF GERBERAPETALS

The effect of vapour pressure deficit, temperature and radiation on th e postharvest susceptibility of gerbera flowers to B. cinerea, on the water relations of gerbera flowers and on the lesion formation after c onidial infection of B. cinerea was studied. The temperature range in which B. cinerea could germinate and grow in vitro is 5-30-degrees-C. In climate chamber experiments flowers had more lesions of B. cinerea at temperatures of 20 and 25-degrees-C than at 10 and 15-degree-C. At 15, 20 and 25-degrees-C the infectivity of B. cinerea conidia was nega tively affected during a storage-period of 7 days. At a vapour pressur e deficit (VPD) of 200 Pa significantly more conidia of B. cinerea wer e infective than at 800 Pa. At a VPD of 800 Pa the susceptibility of g erbera flowers for B. cinerea was not significantly different than at 200 Pa. High radiation levels in glasshouses in spring and summer nega tively influenced the infectivity of conidia of B. cinerea on the flow er surface, but did not affect the susceptibility of gerbera flowers f or B. cinerea. In spring and early summer conidia lost their infectivi ty at high radiation levels, high temperatures and high levels of VPD. In summer gerbera flowers could be more susceptible to B. cinerea bec ause of high temperatures in glasshouses, but the negative effect of r adiation on the conidia of B. cinerea seemed to overrule the temperatu re effect. Thus, the numbers of lesions in spring and summer can be lo w compared with the numbers in other seasons, although the numbers of B. cinerea colonies on spore traps can be high. The effect of temperat ure on the susceptibility of gerbera flowers can probably be explained by changes of water status in the petals. At higher temperatures the number of lesions and the turgor (= water potential - osmotic potentia l) in the petals increased. Temperatures < 10-degrees-C during lesion formation (RH > 95% and VPD < 50 Pa) had a temporary negative effect o n the number of lesions. After 3 days of incubation the numbers of les ions were about equal (greater-than-or-equal-to 30 lesions/cm2) from 5 to 20-degrees-C. At 30-degrees-C no lesion formation was observed eve n after 3 days.