Graduate Thesis Or Dissertation

 

Etiological studies on foot rot of wheat caused by Cercosporella herpotrichoides Fron. Public Deposited

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  • Abundant conidial production occurs from colonized straws and plants infected with Cercosporella herpotrichoides Fron during the cool moist months of the growing season. Experiments were designed to study the survival, inoculum potential and competitive saprophytic ability of conidia in soil. Laboratory results indicated that cool moist soil favors the saprophytic survival of the fungus in soil. Viable hyphae were observed 16 months after conidia were buried in soil held at 5° and 10° C. Ammonium nitrate inhibited germination and survival of conidia in soil. It also inhibited germination on glass slides not in contact with soil. Ammonium ions were more inhibitory than nitrate ions. Glucose did not influence germination but stimulated saprophytic development and survival of the pathogen in soil. A marked reduction in survival of C. herpotrichoides after one year occurred in naturally and artificially colonized straws buried in the soil. Survival was favored in straws placed at the soil surface. Straws buried in conidial infested soil and incubated in the laboratory or the field were colonized by the pathogen. Colonization was directly related to the inoculum density of the soil and inversely related to time. Foot rot lesions developed below the soil surface on plants grown in conidial infested soil. Lesion incidence was directly related to the inoculum density of the soil. Based on mathematical models it was concluded that conidia were influenced by host exudates only at the host surface and a rhizosphere was not operative. The fungus was able to grow up to 10 mm from a food base in the soil and cause below ground lesions. Lesions below the soil surface were observed on plants growing under natural field conditions. It is suggested these below ground infections play an important part in maintaining inoculum levels in fields during years not favorable for foot rot development. In growth chamber studies, plants developed lesions from single conidium inoculations. Disease development on plants in the growth chamber was influenced by the general susceptibility of the inoculated tissue, the stage of growth when inoculated, the rate of plant growth and the openness of the crown. Green tissues were more resistant to infection than senescent tissues. Infection occurring before the onset of tillering can result in lesion development on each new tiller produced. Plants growing rapidly in a high nitrogen media were able to slough off infections and escape the disease. Tight crowned plants tended to have a majority of their tillers infected while healthy culms were associated with diseased ones in plants having wide spreading crowns. Successful penetration and infection of a coleoptile or leaf sheath did not insure disease development. The infected tissues may become separated from the main stem before the fungus has penetrated into adjacent leaf sheaths, thus isolating the pathogen, so that.the majority of the plant tissue escapes infection. The infection process failed at one of three stages on green "resistant" tissues of coleoptiles and leaf sheaths: (1) germination failed to occur, (2) germination occurred but the pathogen failed to attempt penetration, and (3) penetration was attempted but failed. Thickening of host cell walls was associated with the failure of attempted penetrations. Thickened host walls were also associated with containment of the pathogen at the perifery of lesions. These thickened cell walls usually stained heavily with cotton blue and were never associated with senescent tissue. Results indicate that stubble mulching and deep seeding would favor foot rot development. Chemicals or varieties delaying senescence of leaf sheaths would be expected to reduce foot rot injury.
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