Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2007
Publication Date: 10/1/2007
Citation: Ables, C., Rosskopf, E.N., Shah, D., Albano, J.P. 2007. Effect of fertilization, biopesticides, and biorationals on the infection of catharanthus roseus by phytophthora nicotianae. Plant Disease. 91:1477-1483. Interpretive Summary: Phytophthora nicotianae is a serious pathogen of various species of vegetables, fruits, and ornamentals belonging to at least 80 plant genera. Considerable losses in commercial and residential plantings of Madagascar periwinkle (Catharanthus roseus (L.) G. Don, aka ‘Vinca’) have been attributed to this pathogen in the southern part of the United States. Symptoms of Phytophthora blight on periwinkle include the appearance of water-soaked grayish-brown lesions on shoot tips and leaf petioles, girdling of the main stem, wilting, necrosis, and plant death, which may occur within a week or two weeks after the first appearance of symptoms. The pathogen can survive in the soil as chlamydospores or in plant debris and therefore persists from season-to-season unless the soil is disinfested. However, because of restrictions on the use of chemical soil fumigants, other control measures have been employed to control this disease. Growers have primarily relied on fungicides such as metalaxyl to control Phytophthora diseases; unfortunately, the reliance on fungicides is believed to select for resistance in Phytophthora populations. Resistance or insensitivity to metalaxyl has been observed in P. nicotianae isolated from ornamental hosts as well as isolates of other economically important Phytophthora and Pythium species. Restrictions on the use of soil fumigants and the emergence of metalaxyl-resistant or metalaxyl-insensitive populations have shifted the focus of Phytophthora blight control to alternative strategies including the use of biological control agents. Biorationals or reduced-risk chemical pesticides have also been considered for disease control in various crops. These types of pesticides are an attractive option for disease control because they minimize environmental risk by having short residual activity, a high degree of selectivity or a high level of efficacy in small amounts. The application of fertilizers or certain nutrients can reduce disease severity in plants either by directly inhibiting the pathogen or by making the host less susceptible to pathogen attack. This study was done to 1) evaluate and compare the efficacy of some commercial biologically-based pesticides (biopesticides) and biorational products as control agents against Phytophthora root and stem rot on periwinkle under greenhouse culture; 2) determine if fertilization level can enhance the efficacy of these products against Phytophthora nicotianae infection; and 3) determine the effect of fertilization level and inoculum level on the severity of Phytophthora root and stem rot in periwinkle.
Technical Abstract: Experiments were carried out in the greenhouse to determine the effect of fertilizer concentration (0, 0.5, 1.0, and 2.0x Hoagland solutions) and various commercial biopesticides and biorational products on the severity of Phytophthora rot of Madagascar periwinkle. The application of biopesticide and biorational products and the fertilizer concentration significantly influenced the severity of rot, but there was no significant effect from the interaction of these two factors. Overall, Phytophthora rot showed a tendency to increase with the concentration of applied fertilizer. Compared to the control plants, Phytophthora rot severity was significantly lower in plants that were treated with the biopesticides or biorationals, 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 to either the control plants or those that received any of the other products tested. Additional experiments carried out in growth chambers tested the effects of increasing fertilizer concentrations in plants that were inoculated with different P. nicotianae inoculum levels. There was no consistent indication that disease is most severe in plants that received the highest fertilizer concentration or the highest inoculum level.