| Summary: | "Primaries" are traditionally defined as bulbs with foliage already infected when emerging above ground, but the investigation showed that viable propagules of B. tulipae represent the true primary phase of tulip fire disease; traditional primaries reflecting the spread of B. tulipae conidia from primary bulb-borne sources. Bulb development after planting was relatively insensitive to changing temperatures which, however, affected the development of B. tulipae. At 15.5°C inocula of B. tulipae carried on bulbs spread rapidly infecting foliage and sometimes killing the flowering shoot before emergence above ground. At lower temperatures, B. tulipae invaded the subterranean bases of flowering shoots causing changes reflected in shorter brittle shoots with paler green foliage. At 4°C, B. tulipae spread slowly invading developing daughter bulbs without affecting the longevity of the mother plant. Two distinct pathways of spread from mother to daughter bulbs were identified, an inner and an outer. Conidia produced on foliage already infected when emerging above ground were involved in the secondary aerial spread of B. tulipae. Aerially dispersed conidia caused either aggressive or non-aggressive lesions, the former, which ultimately sporulate, being favoured by (a) increasing numbers of conidia/infection drop and (b) prolonged humid periods. Minimising the incidence of foliage lesions, caused by aerially spread conidia, increased bulb yields. Of the fungicides applied at two weekly intervals, mancozeb and dichlo-fluanid formulations gave the best control. Yields were sometimes increased by the same range of fungicides in the absence of B. tulipae, possibly by decreasing the damaging lipolytic activity of 'saprophytic' colonisers of the tulip phylloplane.
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