Reduction in the rate and duration of grain growth in wheat due to stem rust and leaf rust

Leaf and/or stem rust epidemics differing in date of disease onset after anthesis and area under the disease progress curve (AUDPC) were established at various distances from rows of inoculated winter wheat in two fields during each of the 1986 and 1987 growing seasons. Rate and duration of grain growth were reduced by rust. Piecewise linear regression proved to be useful for describing the nonlinear growth curves. Square root-transformed grain dry weight increased linearly at a similar rate (0.19-0.21 mg1/2/day) at each of four field sites for at least the first 12 days from anthesis (DFA). Rust had a greater impact on grain growth in 1986, when stem rust occurred alone, than in 1987, when both leaf and stem rust were present. In 1986, growth rates were inversely related to disease severity in two fields and ranged from 0 .04 to 0.20 and 0.06 to 0.21 mg1/2/day between 15 and 22 DFA and 16 and 23 DFA, respectively, when wheat was in growth stage (GS) 11.1-11.2. Only the most severely infected wheat plants exhibited reduced growth (0.13-0.14 mg1/2/day) during this period in 1987. Growth rates ranged from 0.00 to 0.16 mg1/2/day between 23 and 27 DFA in 1987 when wheat was in GS 11.2-11.3. Grain growth had ceased by 23 DFA for plants with an AUDPC for rust on peduncles greater than about 640 percent-days in both years. Grain growth of plants with low disease severity ceased by 33-36 DFA in 1986 and by 27 DFA in 1987. The rate of grain growth over 5- to 7-day intervals was related to the concurrent average percent green leaf area of flag leaves throughout the grain-filling period. Grain number per head was not affected by rust infection. The amount of dry matter lost from culms during the grain-filling period was not related to disease severity; however, culm dry weight of the most severely rust-infected wheat in each field decreased more rapidly than the culm weight of the other epidemics.