Effect of methyl bromide on the control of Fusarium oxysporum f.sp. dianthi propagules in carnation greenhouse soil and on inoculum increase after one growth cycle of carnation

Abstract

The effectiveness of methyl bromide (80 g m −2) on the control of Fusarium oxysporum f.sp. dianthi ( F. o. dianthi) was studied at soil depths of 0–60 cm, subdivided into three layers of 0–20, 20–40 and 40–60 cm, in five carnation greenhouses during four successive growth seasons. Each growth season, the number of propagules of F. o. dianthi in the soil was counted before and after fumigation. The efficiency of methyl bromide control was also measured in terms of the viability of F. o. dianthi propagules placed in the soil at depths of 0, 10, 20, 30, 40, 50 and 60 cm. Carnation cultivars ( Dianthus caryophyllus) highly susceptible to Fusarium wilt were planted in four of the five greenhouses studied. Moderately resistant cultivars were planted in one greenhouse during three growth seasons and in a second greenhouse during the last growth season. At the end of each growth season, the largest number of propagules (57–100% of total count) was found in the 0–20 cm layer. More inoculum was found in the 20–40 cm than in the 40–60 cm layer. Methyl bromide reduced the F. o. dianthi propagule count at all soil depths between 0 and 60 cm, but was most effective in the upper 30 cm. A count of F. o. dianthi propagules in the soil in fumigated greenhouses on the ten occasions when their level at the end of the previous growth season was high revealed propagule survival rates of 0% (below detection level) five times, 0.1 % twice, 0.6% once, 1.6% once and 4.6% once. The highest count of viable propagules, when methyl bromide was not successful in reducing propagule count to below detection level, was found in the 20–40 cm layer. At the end of each growth season, both inoculum level in the soil and Fusarium wilt incidence varied in the four greenhouses planted with highly susceptible or susceptible carnation cultivars, irrespective of the propagule counts of F. o. dianthi after fumigation. The correlation between inoculum level in the soil after fumigation and inoculum or Fusarium wilt incidence at the end of that growth season was non-significant ( r 2 = 0.001, p = 0.87 for inoculum; r 2 = 0.19, p = 0.2 for wilt incidence). This indicates that in monocultures of susceptible carnation cultivars, methyl bromide cannot give total protection even for one growth season. In each of the two greenhouses planted with moderately resistant cultivars, the number of propagules in the soil at the end of each growth season remained undetectable, as it was after fumigation.