Studies on fairy ring spot of carnations caused by Cladosporium echinulatum : A thesis submitted in partial fulfilment for the requirements for the degree of Master of Horticultural Science Lincoln University (University of Canterbury) New Zealand

Abstract

The causal fungus of fairy ring spot Cladosporium echinulatum (Berkeley) de Vries was isolated into pure culture. Three experiments investigated the growth and conidial production of the fungus on artificial media. At 15°C, V-8 juice agar (containing 10% or 20% V-8 juice) and malt extract agar were optimal for radial growth. The optimum temperature for growth was 15°C on malt extract agar but growth was slow, averaging 1.2mm increase in radius per day. Growth was less on synthetic media. Conidial production was highest on V-8 juice and potato carrot agars and in the light (12D:12L). Few conidia were produced in continuous darkness on these agars. Under greenhouse conditions, 20±5°C, leaves of carnation plants cultivar 'Mei Fu' were inoculated with conidia. The first symptoms were observed 9 days after inoculation (incubation period) ,and conidia began to develop on the leaf surface at 18 days (latent period). Once conidial production commenced ,numbers harvested daily rapidly increased to a mean of 25.1 conidia mm⁻² of infected leaf tissue. This remained relatively constant from day 4 to 16. Small numbers of conidia (0.5 mm⁻² of infected leaf) were still produced 38 days after initial production commenced. Light and scanning electron microscope (SEM) studies showed that the pathogen spread through leaves by intercellular hyphae, with no intracellular haustoria. A mat of mycelium developed in substomatal cavities and conidiophores emerged only through stomata openings. Fixing fresh samples in glutaraldehyde fumes, followed by air drying, gave best preservation of aerial conidiophores and conidia for the SEM studies. Trapping of conidia in a greenhouse showed that air-borne concentrations followed a circadian periodicity with maximum numbers of conidia occurring around midday. These high numbers of conidia were highly correlated with decreasing relative humidity and leaf surface wetness and increasing temperature. High air-borne concentrations of conidia were also associated with overhead watering and the corresponding crop disturbance and fluctuations in relative humidity. Eight experiments evaluated the efficacy of 17 fungicides against C echinulatum. These included in vitro studies, pot trials and commercial greenhouse trials. In vitro, chlorothalonil, maneb, imazalil, pyrifenox and triforine inhibited conidial germination at I00mg l⁻¹ and imazalil, prochloraz. pyrifenox, cyproconizole, flusilazol, hexaconazole, myclobutanil, propiconazole, terbuconazole and carbendazim inhibited radial growth at 1mg l⁻¹. Greenhouse pot trials assessed the protectant and eradicant properties of 13 fungicides. Each fungicide was applied either three days before or three days after inoculation with conidia. Chlorothalonil, maneb and mancozeb were observed to be effective protectants. The site-specific chemicals prochloraz. pyrifenox and CGA 169374 were effective as both protectants and eradicants. Commercial greenhouse trials showed that when disease pressure was high prochloraz, pyrifenox and CGA 169374 provided good disease control as compared to unsprayed plots.