Possible mechanisms influencing the dynamics of rhizoctonia disease of tulips

In two observation fields, where six sites were artificially infested with Rhizoctonia solani AG 2-t, bare patches developed. These patches did not re -occur at the site of infestation in three successive years. In fields with and without artificial infestation, natural infection of tulip bulbs by Rh izoctonia spp. occurred. The spatial distribution of infected tulip bulbs w as visualised in maps after kriging. The influence of sampling intensity wa s evaluated by stepwise reduction obtained in the observed data set of the first year. Omnidirectional semivariogram characteristics did not change wh en sampling intensity was reduced down to 10%. The average maximum predicti on error was minimised at sampling intensities varying from 7% to 25%. Natu rally occurring bare patches slowly vanished during successive cropping of flower bulbs and did not re-appear in the fourth growing season. A high fre quency of isolation of R. solani AG 2-t in one field (Lisse-2) in the fourt h consecutive crop did not result in bare patches in that year. It is hypot hesised that a reduction in aggressiveness may account for this observation . In contrast, bulb rot due to Rhizoctonia spp. increased during the observ ation period. R. solani AG 5 isolates were seldom isolated before the bulbs flowered, but were the dominant isolate from bulbs at harvest. In a growth chamber experiment, it was demonstrated that AG 5 did not account for repl acement of AG 2-t. However, it was demonstrated that competition may partia lly explain replacement of AG 2-t isolates during the growing season. At 18 degreesC, but not at 9 degreesC, an AG 4 isolate prevented AG 2-t colonisi ng and infecting iris bulbs when both isolates were introduced together to soil. Rhizoctonia populations develop in relation to soil temperature and p lant development. It is hypothesised that a 'temporal niche differentiation ' may be one of the mechanisms affecting the dynamics of rhizoctonia bare p atch of tulips.