Bell peppers and chile peppers are important crops in many regions of the U. S. In most areas where peppers are grown, the risk of phytophthora pepper blight is always present. This disease is caused by the soil-borne pathogen Phytophthora capsici.
In Arizona, the root and crown rot phase of the disease can appear on plants early in the growing season in areas of fields that do not drain well and remain saturated after an irrigation. Subsequent irrigations encourage further disease development. As the crop canopy covers the rows within the field, the foliar blight phase of the disease may occur with the onset of the summer rainy season in July and August. During this time, stems, leaves, and developing pepper fruits may be attacked by the pathogen.
Losses due to phytophthora pepper blight can be minimized by proper water management, use of resistant cultivars, and application of the fungicide Ridomil Gold; however, water in the form of rainfall is not manageable, resistant cultivars may not be available, and insensitivity to Ridomil Gold has been detected in some pepper growing areas.
Fungicide need There is an urgent need for efficacious fungicides in addition to Ridomil Gold to prevent disease control failure in pepper fields subjected to high disease pressure during prolonged periods of plant wetness and saturated soil conditions, and to minimize the development of resistance to this chemistry by Phytophthora capsici.
Studies were initiated at the University of Arizona, Yuma Agricultural Center to evaluate and compare the activity of Ridomil Gold to four nonregistered fungicides (azoxystrobin, dimethomorph, fosetyl-Al, and fluazinam) with respect to their ability to suppress growth, sporulation, and disease initiation by Phytophthora capsici.
The ultimate value of any chemical compound as a control agent for a disease depends on the ability of the chemical to stop spore production or growth of the fungus. For phytophthora, sporangium formation and subsequent release of zoospores provide the greatest opportunity for a rapid buildup in the number of infective zoospores and resultant higher potential for infection and disease development. For this reason, laboratory studies were conducted to compare the ability of each fungicide to suppress sporangium formation.
Dimethomorph, fluazinam, or Ridomil Gold completely inhibited sporangium formation at a concentration of 10 mg active ingredient (a. i.) per liter of water. A 1,000 mg a.i./ liter concentration was necessary to achieve complete suppression of sporangium formation with azoxystrobin and fosetyl-Al.
Once plant infection has occurred, suppression of mycelial growth within host tissue becomes an important disease management consideration. Again, laboratory tests revealed that mycelial growth by Phytophthora capsici was virtually stopped by dimethomorph and Ridomil Gold at a concentration of 100 mg a.i./liter. At 1,000 mg a.i./liter, fosetyl-Al prevented mycelial growth; however, growth was only partially suppressed at this concentration by azoxystrobin and fluazinam.
Disease control was evaluated by planting a chile pepper plant into a series of pots containing soil collected from a field with pepper plants infected with Phytophthora capsici. After planting, the soil in each pot was drenched with various concentrations of each tested fungicide or left untreated as an infested control.
For a noninfested soil control, soil was steam-sterilized before planting to peppers. Half of the plants were irrigated every two to three days to represent normal application of water to plants in the field, while the other half of the plants were irrigated in the same manner except for a two-day flood period every two weeks to produce occasional saturated soil conditions, which in the field promote rapid development of phytophthora crown and root rot.
Pepper plants were maintained in the greenhouse for two months, after which final disease measurements were made. Under both irrigation regimes, planted treated once with any of the five tested fungicides survived for a significantly longer time period than plants grown in nontreated infested soil.
When irrigated every two to three days, survival of plants treated with dimethomorph or Ridomil Gold at 100 mg a.i./1 or fluazinam at 1,000 mg a.i./1 was not significantly different than plants grown in sterilized soil. When flooded for two days every two weeks, survival of plants treated with dimethomorph and fluazinam, but not Ridomil Gold, did not differ significantly from plants grown in sterilized soil.
Study findings Our studies have shown that a soil application of azoxystrobin, dimethomorph, fosetyl-Al, or fluazinam, as well as Ridomil Gold, can significantly extend the duration of plant survival under conditions highly favorable for disease development. Among these fungicides, dimethomorph and fluazinam provided the highest level of protection against phytophthora crown and root rot.
If one or more of these nonregistered fungicides would become available for use on peppers in the future, treatment programs utilizing fungicides with different modes of action could be implemented. Such a development could prolong the effectiveness of these materials for management of disease.