Graduate Thesis Or Dissertation
 

Factors affecting development of iris rhizome rot caused by Botrytis convoluta Whetzel and Drayton

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  • Abundant conidial and sclerotial production occurs on iris plants infected with Botrytis convoluta Whetzel and Drayton during the cool moist months of the year. Experiments were designed to study the survival and inoculum potential of conidia and sclerotia. Basic nutritional requirements of the fungus in culture were also studied. Results of field and laboratory studies indicated a large percentage of iris plants apparently free of B. convoluta carried latent infections. The evidence indicates that these latent infections originate from contact of healthy tissue with senescent or dead leaf and/ or rhizome tissues. Progression into healthy tissues is halted by increasingly higher soil and air temperatures and periderm formation which walls off the infections. Active rot development occurs when conditions favorable for pathogenesis return. These latent infections which are undetectable visually would be a very important means of dissemination of the disease since growers believe they are shipping sound rhizomes. Chemical control would be difficult because the infections are inaccessible to non-systemic fungicides. Field inoculation of rhizomes with conidia, sclerotia and rolled-oat cultures of B. convoluta resulted in significantly increased infection of iris plants inoculated with conidia. Sclerotia placed onto wounded areas of rhizomes also caused significantly increased infection incidence. Differences in field resistance to the pathogen were also noted. Iris plants of the variety Cotlet were found generally more prone to infection and more extensively rotted than Apricot Glory and Sunset Blaze varieties. Laboratory results indicated that conidia and sclerotia were able to cause infection of intact iris rhizome offsets. Conidia applied to offsets colonized senescent leaves and, using this as a food base, invaded healthy juvenile leaves in contact with them. Germ tubes were observed lying parallel to and over longitudinal leaf epidermal cell wall junctures and in several instances, they appeared to be between cells. Terminal appressorium-like swellings of germ tubes were observed on leaf surfaces; however, no direct penetration of leaf tissue was observed. Sclerotia remained viable for one year at 5 to 25°C under dry storage conditions. Survival of conidia stored in situ on colonies for 257 days decreased with increasingly higher temperatures, dropping to less than five percent at 25°C after 210 days. Survival of sclerotia or conidia was not markedly affected by storage at -70°C for 90 days, and conidia remained at least 70 percent viable at -70°C after 257 days. Sclerotia survived in moist soil for 60 days at temperatures of 15°C or below. No sclerotia were found viable after 60 days when soil temperatures were above 20°C. Non-viable sclerotia were found to be colonized by other fungi; mainly species of Trichoderma, Fusarium, Mucor, and Sepedonium as well as by bacteria. Nutritional studies in vitro showed B. convoluta to utilize the carbohydrates maltose, glucose, sucrose, starch, galactose, and fructose, from good to poor carbon sources respectively. Lactose and sorbose were poor carbon sources. Nitrogen sources, from good to poor, were casein hydrolysate, asparagine, ammonium tartrate, ammonium sulfate, and glutamate. Potassium nitrate, glycine, and urea were poor nitrogen sources. Growth was most rapid when the pH of the media was pH 3-4. B. convoluta required light to be stimulated into sporulation. Sporulation was stimulated by exposure to white or to near-ultraviolet light. Optimum temperatures for linear growth was near 23°C and optimum for sclerotium formation was 10-15°C. Germination of conidia incubated in distilled water was generally very low (less than 10%) and germination was invariably much higher (85-100%) when an exogenous carbon source was supplied.
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