Effect of Fertilization and Biopesticides on the Infection of Catharanthus roseus by Phytophthora nicotianae.

Experiments were carried out in a greenhouse to determine the effect of fertilizer concentration (0, 0.5, 1.0, and 2.0× Hoagland solutions) and various commercial biopesticides on the severity of Phytophthora nicotianae infection of Madagascar periwinkle. Application of biopesticides and fertilizer concentration significantly influenced the severity of infection, but there was no significant effect from the interaction of these two factors. Overall, disease severity showed a tendency to increase with the concentration of applied fertilizer. Compared with the control plants, disease was significantly less severe in plants that were treated with the biopesticides, except for plants treated with metabolites of Myrothecium verrucaria (DiTera). However, only the products containing potassium phosphonates and potassium phosphates (FNX-100 and FNX-2500) provided a satisfactory level of control when compared with either the control plants or those that received any of the other products tested. Additional experiments were carried out in growth chambers to test the effects of increasing fertilizer concentrations in plants that were inoculated with different P. nicotianae inoculum levels. In these trials, there was no consistent indication that disease is most severe in plants that received the highest fertilizer concentration even at the highest inoculum level.