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Potato is an important crop globally and late blight (Phytophthora infestans) often results in severe crop loss. The cost for late blight control can be in excess of $210 million in the United States. We utilised a non-parametric density distribution analysis of local temperature (T) and relative humidity (RH), from 2005 to 2009, to assess and validate sporangia survival potential using survival model and late blight risks during the potato cropping season at Presque Isle, in the northern part of the state of Maine, USA. Modelbased analyses showed that ambient temperatures of 3−30°C and RH values of 45−100% were conducive for sporangia survival. Disease outbreaks and risk periods coincided with a high sporangia survival probability (15−35%). Due to the omission of solar radiation (SR) in the computation of survival potential in previous research, we applied a Cox proportional model to estimate the probability of sporangia survival [i.e. hazard at a specific time H(t)] as a function of baseline hazard (H0) and the influencing parameters. The model is: H(t) = H0(t) × exp(0.067ET + 0.138T + 0.083RH + 0.001SR) where ET is exposure time. The survival model indicated that RH (β = 0.083) and T (β = 0.138) were significant (p < 0.05) factors in sporangia survival in comparison to SR (β = 0.001). The hazard ratio, indicative of sporangia survival risk, varied with the predictors. For the unit increase of T, sporangia survival hazard increased by 1.148 times. The Cox model and sporangia hazard probabilities can be used for short-term disease forecasts based on the risk period most conducive for pathogen survival and targeted fungicide applications for optimum late blight management.
The present study was conducted to illustrate the micromorphology of mature sporangia and pollen grains of four Ginkgo biloba cultivars: ‘Horizontalis’, ‘Lasiocarpa’, ‘Pragensis’ and ‘Tubifolia’ and is a continuation of earlier investigations on this subject. The sporophylls bear typically two pendulous sporangia, whereas three were found in one cultivar. The sporangia varied in shape from oval, elliptical elongate to boat-shaped and dehisced along a longitudinal slit. The sporangial walls are multilayered. The sculpture of the sporangium is rugose covered with wax crystals (tubules) and the endothecium shows clearly visible fibrous thickening on the wall. The pollen grains are monosulcate, the non-apertural surface is irregularly ridged (striate) or rugulate, with perforations and the germinal aperture (furrow) is covered with verrucae. The current study did not differentiate significantly four cultivars based on sporangia and pollen morphology. The micromorphological traits may be useful for identification and classification of fragments of fossil sporangia and pollen grains.
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