Effects of paclobutrazol on Botrytis cinerea isolates obtained from potted plants

The growth of different isolates of Botrytis cinerea, collected from potted plants affected by Botrytis blight in southern Spain during recent years, was studied. These isolates, which show wide phenotypic differences when grown in vitro, are differentially affected by growth temperature, gibberellic acid, and paclobutrazol--an efficient plant growth retardant used widely in nursery potted plants to reduce plant size, favouring compactness, a more intense green foliage and increased stress tolerance to maintain quality prior to sale. In addition, paclobutrazol may have a fungicidal effect since it belongs to the triazole chemical group. However, paclobutrazol is only used as a plant growth retardant in Spain. In this work, we evaluate the effect of paclobutrazol dose (0, 0.05, 0.25, 1.25, and 6.25 mg/plate) on the growth of a collection of different B. cinerea isolates obtained from the following potted plants: Cyclamen persicum, Hydrangea macrophylla, Lantana camara and Lonicera japonica. Mycelial growth curves and growth rates assessed from difference in colony areas during the linear phase, conidiation (measured as time of appearance), conidial length (microm), and sclerotia production (number/plate) were evaluated in the isolates, which were grown at 26 degrees C on Petri dishes containing potato dextrose agar for up to 36 days. Mycelial growth curves fitted a typical kinetic equation of fungus grown on solid media. The B. cinerea isolates showed a high degree of variability in their growth kinetics, depending on the isolate and paclobutrazol dose. This triazole delayed mycelial growth during the linear phase in an isolate-dependent manner, and isolates from C. persicum and L. japonica were more affected by paclobutrazol than H. macrophylla. On the other hand, 0.25 mg of paclobutrazol was the critical dose to significantly reduce the growth rate in all isolates. 6.25 mg paclobutrazol inhibited conidiation in isolates from C. persicum, and reduced the conidial length in isolates from H. macrophylla and L. camara. The sclerotia production process was blocked at paclobutrazol doses higher than 1.25 mg, while no sclerotia were produced in isolates from C. persicum and L. japonica with 0.25 mg. H. macrophylla was the isolate in which sclerotia production was most influenced by paclobutrazol. It was concluded that the exact effect of paclobutrazol on B. cinerea growth depends on the isolate, and new strategies should be considered for evaluating its use as retardant and fungicide.