Colonies of Botrytis cinerea were obtained from spore samplers placed inside and outside a glasshouse with a rose crop. Pure cultures were made from five colonies collected on one sampling date every month throughout the year. These isolates were tested for germination on water agar and for pathogenicity on gerbera and rose petals. The germination rate of the conidia on water agar varied between 60 and 99%. Pathogenicity of the isolates on gerbera and rose flowers ranged from 14 to 166% relative to reference isolate Bc16 and varied among isolates collected on the same day as much as among isolates collected in different months. The pathogenicity of the isolates on rose flowers was overall higher than on gerbera flowers. Random amplified polymorphic DNA (RAPD) analysis was performed on 30 selected isolates with different relative pathogenicity, collected both inside and outside the glasshouse. Almost all of the isolates were genetically different. No correlation was found among pathogenicity, sampling time, sampling place, and RAPD patterns. Results are further evidence for the statement that B. cinerea inoculum in glasshouses continuously originates from many different sources in their vicinity.
A selective medium has been developed for the use in spore-traps to study the dispersion of B. cinerea on gerbera grown in glasshouses.Additional keywords: dispersal, gerbera.To study the dispersion of air-borne fungi in glasshouses different types of spore-traps were used (Frinking et al., 1987;Hirst, 1959;Jarvis, 1962). These traps were based on air-suction or on impaction on sticky surfaces by natural airflows. In the case presented here fungal spores in glasshouse air were trapped using Petri dishes containing an agarbased culture medium to study the dispersion of the air-borne spores of B. cinerea on gerbera grown in glasshouses.In glasshouse air, spores of many different fungi are present. When an universal medium was used, for example Potato Dextrose Agar (PDA), it was very difficult to recognize colonies of specific fungi such as Botrytis cinerea. Within 24 hours of incubation fast growing fungi such as Mucor spp. covered the whole surface of the universal medium, suppressing other fungi. For this reason it is necessary to use a selective medium in the spore-trapping device.A selective medium for B. cinerea was described by Kritzman and Netzer (1978). They developed their medium for the isolation and identification of Botrytis spp. from soil and onion seed. The medium contains tannic acid as a substrate for polyphenoloxidase (PPO) activity. PPO activity of growing mycelium of a culture of Botrytis allii was found to convert the tannin in the medium into a dark-brown pigment. Other fungi were found to have PPO too, but were inhibited by PCNB (pentachloronitrobenzene), Maneb and CuSO4. Germination of conidia, hyphal elongation and mycelium growth ofB. allii and B. cinerea were not prevented on the selective medium. Brown pigmented colonies could be identified 48 hours after incubation at 24 ~ (Kritzman and Netzer, 1978). This selective medium is likely to be very useful for the study of the development of B. cinerea.The following selective medium for B. cinerea is an adjustment of the selective medium used by Kritzman and Netzer. The basal medium, which was almost the same as that used by Kritzman and Netzer, consists of the following components (g/1 distilled water): NaNO3, 1.0; K2HPO4, 1.2; MgSO4.7H20, 0.2; KC1, 0.15; glucose, 20.0 and agar, 25.0. The medium was sterilized for 20 minutes at 121 ~ After cooling ti165 ~ the follow-247 ing ingeredients were added (g/1 distilled water): Terrachlor (PCNB, pentachlorobenzene 75~ WP), 15 • 10 3; Maneb (manganese ethylene bisdithiocarbamate), 1 x 10-2; chloramphenicol (antibiotic), 5 x 10-2; CuSO4, 2.2; Rubigan (fenarimol), 0.1 ml/1 and tannic acid, 5.0. The pH of the supplemented basal medium (SBM) was adjusted to 4.5 with 5.0 N NaOH. Rubigan was added into the selective medium to suppress airborne spores of Penicillium spp. At pH 4.5 colonies of B. cinerea formed mycelium, conidiophores and conidia. At pH 6.5, used by Kritzman and Netzer (1978) B. cinerea was not able to form conidia.Germination and mycelial growth of B. cinerea was not prevented on thi...
Dispersal of Botrytis cinerea in a gerbera crop grown in two glasshouses 30 km apart was studied over a period of 18 months, in 1988 and 1989. Conidia were caught in spore traps consisting of agar in petri dishes exposed at different heights in the crop in each glasshouse. No seasonal patterns could be identified in the spore catches, assessed as colonies on the agar traps after incubation. The number of lesions caused by conidial infection of gerbera flowers following incubation, however, showed a distinct pattern. In spring and early summer few lesions were recorded whereas at other times of the year many lesions appeared. In linear regression analysis, variation in numbers of colonies (spore catches) could not be explained by environmental factors recorded during the experiments. Linear regression accounted for 77% and 81% of the variation in the number of lesions on flowers in the two glasshouses, in terms of relative humidity (postively correlated), global radiation outside the glasshouse (negatively correlated) and age of the crop (positively correlated). Despite differences in the systems by which the gerbera crop was produced and in the spore catches, the numbers of lesions on gerbera flowers in the two glasshouses were significantly correlated though not significantly different from each other.
The effect of vapour pressure deficit, temperature and radiation on the postharvest susceptibility of gerbera flowers to B. cinerea, on the water relations of gerbera flowers and on the lesion formation after conidial infection of B. cinerea was studied. The temperature range in which B. cinerea could germinate and grow in vitro is 5-30 ~ In climate chamber experiments flowers had more lesions of B. ctnerea at temperatures of 20 and 25 ~ than at 10 and 15 ~ At 15, 20 and 25 ~ the infectivity of B. cinerea conidia was negatively affected during a storage-period of 7 days. At a vapour pressure deficit (VPD) of 200 Pa significantly more conidia of B. cinerea were infective than at 800 Pa. At a VPD of 800 Pa the susceptibility of gerbera flowers for B. cinerea was not significantly different than at 200 Pa. High radiation levels in glasshouses in spring and summer negatively influenced the infectivity of conidia of B. cinerea on the flower surface, but did not affect the susceptibility of gerbera flowers for B. cinerea. In spring and early summer conidia lost their infectivity at high radiation levels, high temperatures and high levels of VPD. In summer gerbera flowers could be more susceptible to B. cinerea because of high temperatures in glasshouses, but the negative effect of radiation on the conidia of B. cinerea seemed to overrule the temperature effect. Thus, the numbers of lesions in spring and summer can be low compared with the numbers in other seasons, although the numbers of B. cinerea colonies on spore traps can be high. The effect of temperature 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 potential) in the petals increased. Temperatures < 10 ~ during lesion formation (RH > 95% and VPD < 50 Pa) had a temporary negative effect on the number of lesions. After 3 days of incubation the numbers of lesions were about equal (> 30 lesions/cm 2) from 5 to 20 ~ At 30 ~ no lesion formation was observed even after 3 days.
Quantification and horizontal distribution of air-borne inoculum of Botrytis cinerea in a rose crop in a glasshouse of 300 m 2 was studied in 1991 and 1992. Conidia of B. cinerea were caught in spore traps consisting of an agar medium selective for B. cinerea in Petri dishes placed within the crop, at flower height 1 m above the ground. Spore catches were counted as colonies, after incubation. Lesions due to conidial infection were counted on petals of rose flowers, also after incubation. Relative humidity (RH) and temperature within the glasshouse and global radiation and windspeed outside were recorded during the experiments. The horizontal distribution of B. cinerea in a rose crop grown under glass was fairly uniform in both years. In 1991 a clear seasonal pattern in the number of colonies could not be found. In 1992 the number of colonies were high in August, September and October. The number of lesions on rose flowers showed a distinct pattern in both years. In August, September and October many lesions were counted whereas in the other months few lesions appeared. In linear regression analysis, variation in numbers of colonies (spore catches) could not be explained by environmental factors recorded during the experiments. Linear regression accounted for 76 and 63% of the variation in the number of lesions on rose flowers in 1991 and 1992, in terms of relative humidity (positively correlated), global radiation outside the glasshouse (negatively correlated) and numbers of colonies on spore traps (positively correlated). The results in the rose crop suggest that RH, global radiation and spore density in glasshouses are important variables in regulating the numbers of lesions during storage and transport. The numbers of spores in glasshouses are dependent on the production system. A glasshouse with a system resulting in wet dead tissue on the ground give higher amount of spores in the glasshouse air and through that high numbers of lesions on flowers. On roses outside the glasshouses very high numbers of lesions were counted sometimes, mostly during and after rain showers, as a result of rain-deposition of spores onto the flowers.
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