The influence of temperature, inoculum level and race of Fusarium oxysporum f.sp. cubense on the disease reaction of banana cv. Cavendish

Cavendish banana plants were grown in soil infested with different levels of microconidia of races 1 and 4 of Fusarium oxyspomm Schlect. f. sp. cubense (E.F. Smith) Snyd. & Hans. The 4-week-old plants were incubated at 20¦ and 28¦C in temperature controlled cabinets subject to natural light fluctuations. Race 1, which does not normally infect Cavendish in the field, infected the plants at both temperatures, as did race 4. Results indicated that temperature was primarily affecting plant growth rather than influencing the aggressiveness of the pathogen. Inoculum levels were also found to influence disease severity with a minimum of 4.5 x l04 microconidia/g dry weight of soil required before macroscopic symptoms were observed. The suitability of month-old tissue culture derived plants for screening for resistance and the evolution of the Cavendish attacking race 4 strains are discussed in the light of these findings

Genetic Relationships of Crown Rust Resistance, Grain Yield, Test Weight, and Seed Weight in Oat

Integrating selection for agronomic performance and quantitative resistance to crown rust, caused by Puccinia coronata Corda var. avenae W.P. Fraser & Ledingham, in oat (Avena sativa L.) requires an understanding of their genetic relationships. This study was conducted to investigate the genetic relationships of crown rust resistance, grain yield, test weight, and seed weight under both inoculated and fungicide-treated conditions. A Design II mating was performed between 10 oat lines with putative partial resistance to crown rust and nine lines with superior grain yield and grain quality potential. Progenies from this mating were evaluated in both crown rust-inoculated and fungicide-treated plots in four Iowa environments to estimate genetic effects and phenotypic correlations between crown rust resistance and grain yield, seed weight, and test weight under either infection or fungicide-treated conditions. Lines from a random-mated population derived from the same parents were evaluated in three Iowa environments to estimate heritabilities of, and genetic correlations between, these traits. Resistance to crown rust, as measured by area under the disease progress curve (AUDPC), was highly heritable (H = 0.89 on an entry-mean basis), and was favorably correlated with grain yield, seed weight, and test weight measured in crown rust-inoculated plots. AUDPC was unfavorably correlated or uncorrelated with grain yield, test weight, and seed weight measured in fungicide-treated plots. To improve simultaneously crown rust resistance, grain yield, and seed weight under both lower and higher levels of crown rust infection, an optimum selection index can be developed with the genetic parameters estimated in this stud