Scheduling

Abstract

Two fungicides registered for the control of Pythium spp. were evaluated for their effects on size and time to flowering of seed-propagated geraniums (Pelargonium × hortorum L.H. Bailey). Fungicide drenches of fenaminosulf and metalaxyl were applied to geraniums grown in soilless root medium: 1) at seeding (S); 2) at seeding and transplanting (ST); 3) at seeding, transplanting, and 1 week after transplanting (ST + 1); 4) at transplanting (T); and 5) 1 week after transplanting (T + 1). Metalaxyl drenching schedules did not significantly influence plant size or time to flowering. Fenaminosulf drenching schedules 3 and 4 significantly reduced plant size, and drenching schedule 3 significantly increased days to flowering in comparison to control plants. Although fenaminosulf is used infrequently because of limited availability, the detrimental effects of this fungicide on plant size and time to flowering warrant similar investigations with additional fungicides and crops. Chemical names used: sodium[4(dimethylamino) phenyl]diazenesulfonate (fenaminosulf); N-(2,6-dimethylphenyl) -N(methoxyacetyl) -DL-alanine methyl ester (metalaxyl). Pythium spp. are important and often devastating pathogens of seed-propagated geraniums. Root rot and lower stem rot of young and mature geraniums caused by Pythium ultimum Trow can be a common occurrence among certain cultivars, causing considerable plant loss (Hausbeck et al., 1987; Powell, 1982). Less severe root rot may result in plant stunting and delay of flowering that may go unnoticed if silver thiosulfate (STS), a petal-shattering preventative, is not applied (Hausbeck et al., 1989a). Silver thiosulfate application to seed-propagated geraniums infected with even low levels of P. ultimum increases the incidence of plant death caused by pythium lower stem rot (Hausbeck et al., 1989b). Current control practices for pythium root rot and lower stem rot of seed-propagated geraniums include sanitation and application serve as inoculum that may be unknowingly transported throughout production areas by greenhouse personnel (Stephens et al., 1983; Stephens and Powell, 1982). Several fungicides are recommended for control of diseases caused by Pythium spp. on seed-propagated geraniums (Powell, 1982; Stephens, 1984). Preventive fungicide treatment is warranted for production systems using STS but invariably involves fungicide applications to plants not infected with the targeted pathogen. The effects of such drenches on plant growth and time to flowering are seldom considered in disease management strategies, primarily due to lack of information. The objective of this study was to determine the effects of two fungicides registered to control Pythium spp. on the size and time to flowering of seed-propagated geraniums. pleted, the crop coefficients used in the USDA irrigation scheduling program were revised. This was done by trial and error, until resimulations with 1987 weather data showed that the program would predict an irrigation regime similar to that applied to T, in 1987. An equation for computing these revised coefficients is given in Table 4 and the revised coefficients are shown in Figs. 1 and 2. Note that the revised coefficients would indicate early season Kco values < 0.30. However, a statement in the scheduling program specifies 0.30 as the minimum value of Kco. With the revised coefficients, it is also necessary to assume that the root-zone depth increases linearly from 30 cm at planting to 60 cm at effective cover and it is necessary to specify irrigations when 40% of the available soil water in the root zone is depleted. Use of the revised coefficients in 1986 would have closely duplicated T2, the treatment that produced a high yield and the best WUE. In 1985, the revised coefficients would have given an irrigation regime similar to T4, which produced the highest, although not statistically significant, yield and WUE. Table 5 lists the number of irrigations and depth of water applied to the best treatment in each of the 3 years, and, for comparison, the same data if the revised crop coefficients and rooting assumptions had been used. We can conclude that the revised coefficients would have produced the best irrigation scheduling results for all 3 years of our study. We recommend an irrigation scheduling program based on the revised coefficients (Table 4) for carrots in northern Colorado and similar climates to obtain the best combination of high yields and water use efficiency with a minimal number of irrigations. The irrigation scheduling program is available from E. G. K., J. E. E., and A.E.M. for use on IBM-compatible microcomputers.