Initiation and spread of footrot of wheat caused by Cercosporella herpotrichoides Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rr172183n

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  • Footrot of wheat (Cercosporella herpotrichoides Fron) is a major disease of winter wheat in certain areas of the world having a cool, damp winter and early spring climate. In Oregon, it is an economic problem in the higher rainfall wheat-growing areas in the eastern portion of the state. This study was undertaken to obtain basic information concerning inoculum production, initiation of infection and epidemic development of this disease. The activity of the fungus is strongly controlled by its microclimate, requiring cool, damp conditions for optimal activity. Temperature is the primary factor regulating inoculum production, as abundant moisture is not usually a limiting factor under field conditions from late fall through early spring. Maximum sporulation occurred at 10 C, decreased to insignificant amounts above 20 C and ceased below 0 C and above 30 C. Under fluctuating temperatures, abundant sporulation continued when day temperatures were in the optimal range (8-12 C), as long as the duration of sub-0 C night temperatures (-5 C) did not exceed 14 hrs. A method of evaluating fluctuating temperature regimes was developed by calculating Daily Thermal Sporulation Coefficients (DTSC). The amount of sporulation is a function of the total hours of favorable and unfavorable temperatures that occur daily as reflected by DTSC values. Active sporulation periods are defined as periods of at least 2-3 weeks duration in which the humidity near the soil remains near saturation, the air temperature is above freezing for more than 8 hrs /day, and the average DTSC is above 50. Maximum sporulation levels can be maintained continuously up to 50 days under these conditions. Sporulation periods were identified from field temperature data and used to assess seasonal epidemic potential. The temperature range over which infection can occur is identical to that for sporulation. Infection requires free moisture and is stimulated by the presence of exogenous nutrients, while continued high humidity favors lesion development. All aerial plant tissues, including leaf blades, are susceptible to infection if these requirements are met; susceptibility increasing with tissue age. Infection occurs naturally at the base of the plant by colonization of senescent leaf sheaths, enabling the fungus to establish a food base and increase its infection potential. Rapid colonization of these tissues also provides a protected infection court free from moisture stress, enabling the fungus to overcome the resistance of underlying green tissues. Movement of infection from point sources of inoculum was evaluated in the field. Slopes of disease gradients calculated by plotting the log percent infection against the log distance from the inoculum source were less than slopes calculated from a rain splash model system and decreased throughout the growing season. The effective dispersal range of the fungus was only 3-4 feet. Plants located 3 feet from the inoculum source showed 50% infection 88 days later than those located 1 foot away. Apparent infection rates (r) were low (.005-.011) as compared with cereal rusts (. 100 -. 500). An increase in r was noted in mid-spring in plots with inoculum but not in plots where the inoculum was removed. This indicates that early spring as well as late fall is an important infection period. Analysis of the data fits Van der Plank's "simple interest" model which suggests no role for secondary inoculum in the disease cycle. This is supported by field observations showing abundant sporulation on stubble and negligible amounts on new lesions.
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