The response of cotton to constant salinity has been well documented under controlled conditions, but its response to changing salinity under field conditions is poorly understood. Using a split‐plot design, we conducted a 2‐year field experiment to determine the effects of soil salinity and plant density on plant biomass, boll load, harvest index and leaf senescence in relation to cotton yield in three fields with similar fertility but varying salinity. The main plots were assigned to weak (electrical conductivity of soil saturated paste extract, ECe = 5.5 dS m−1), moderate (ECe = 10.1 dS m−1) and strong (ECe = 15.0 dS m−1) soil salinity levels, while plant density (3.0, 4.5 and 7.5 plants m−2) was assigned to the subplots. Soil salinity had a negative effect on seedcotton yield, but the negative effect was compensated for by increased plant density under strong‐salinity conditions. Seedcotton yield under weak salinity changed little with varying plant density, but the medium plant density yielded better than the low or high plant density under moderate salinity. Plants accumulated 49 and 112 % more Na+ in leaves under moderate and strong salinity than under weak salinity. Strong salinity also led to higher boll load and early leaf senescence. Plant density had no effect on Na+ accumulation in leaves, but greatly reduced boll load and delayed leaf senescence. Plant biomass, maximum leaf area index and harvest index were greatly affected by salinity, plant density and their interaction. Accelerated leaf senescence under strong salinity was attributed to the high boll load and increased accumulation of toxic ions like Na+ in leaves, while delayed leaf senescence with increased plant density was attributed to the reduced boll load. Optimal yield can only be obtained with proper coordination of total biomass and harvest index by modification of plant density based on salinity levels.

Previous studies have shown that the dead mycelium of Penicillium chrysogenum at 900-1,500 kg ha–1 effectively  controlled Fusarium and Verticillium wilt of cotton. Our objective was to determine if the dead mycelium is also  effective at lower rates under a cotton seedling transplanting system. Columned soil blocks (500 g each), made of  fertile soil alone in the first experiment, and both soil and dead mycelium at varying rates in the second experiment,  were sown with cotton seeds in 2006 and 2007. Seedlings in the mycelium-free blocks were drenched with aqueous  extract of the dead mycelium after full emergence, and then transplanted to a saline field. The disease severity of  Fusarium oxysporum f.sp vasinfectum and Verticillium dahliae, leaf chlorophyll and malondialdehyde concentrations,  leaf photosynthetic rate and lint yield of the transplanted plants were determined. The aqueous extract at a rate of 1  to 5% provided 18.6 to 25.6% protection against Fusarium wilt but not against Verticillium wilt. Lint yield was slightly  increased with the aqueous extract in 2006, but not in 2007. The incorporation of the dead mycelium to soil blocks  before sowing was effective in controlling both the Fusarium and Verticillium wilts of cotton. Dead mycelium applied  at 1 to 3% (w/w) provided 26-30% protection against Fusarium wilt and 48-50% protection against Verticillium wilt,  and increased lint yield by 13-14%. Such incorporation also delayed leaf senescence as indicated by the increased leaf  photosynthetic rate and chlorophyll content, and reduced malondialdehyde concentrations. Incorporation of the dead  mycelium of P. chrysogenum into soil blocks at a relatively lower rate (270 kg ha–1) would be an effective application  mode for wilt control in sustainable cotton production.

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