Predicting epidemics of yellow rust (Puccinia striiformis) on the upper canopy of wheat from disease observations on lower leaves

Abstract

Data from field experiments were used to test whether disease observations on lower leaves of wheat were good predictors of future epidemic development in the upper canopy. Fungicide treatments to replicated plots in 1994 and 1995 caused variation in levels of initial inoculum of urediniospores of yellow rust ( Puccinia striiformis ). Observations of symptom severity were made on individual leaf layers throughout the season. Sporulating lesions on lower leaves were considered to be a measure of inoculum source strength for transfer to upper culm leaves. Low source strengths were associated with delays in epidemic development on the upper leaves, as quantified by a location parameter, t m , the time at which severity reached half of the asymptote value in logistic fits to disease progress data on the upper leaves. However, there was much unexplained variation, probably due to extraneous variation in upward transfer efficiency of inoculum and rates of epidemic development. Vanderplank's sanitation ratio theory was used to account for variation in inoculum transfer and epidemic rate. The analysis revealed that if the aim is to predict disease on a newly emerged culm leaf, during the period when it can be treated effectively with fungicide, then observations of disease two leaves further down the culm were of the greatest predictive value ( R 2 = 86%). These observations, however, need to be integrated with information about factors affecting upward inoculum transfer and rates of epidemic development if acceptable predictive precision is to be achieved.

The importance of weather and agronomic factors for the overwinter survival of yellow rust (Puccinia striiformis) and subsequent disease risk in commercial wheat crops in England

“…The significant interaction of number of fungicide applications × number of frosts in Model 1 suggests that reductions in overwintering inoculum resulted in more effective control of yellow rust control. The most likely explanation is that a given number of fungicide sprays was more effective against epidemics that were delayed in their onset by reduced initial inoculum (Young & Paveley, 2003). However, the interaction was not significant in Model 2 and potential savings in the number of fungicide applications used for yellow rust control after a cold winter may be small.…”

“…The model developed in this study provides new opportunities to guide control of yellow rust in wheat crops by quantifying seasonal risk. This could be used with information of cultivar resistance, prevalence of yellow rust races identified through pathogen virulence surveys (Bayles et al , 1989) and observations of disease during the growing season (Audsley et al , 2005; Young & Paveley, 2003) to guide use of fungicides to protect the upper leaves that are crucial to yield formation.…”

“…in the UK are generally associated with the emergence of new races of the pathogen capable of overcoming the resistance of one or more widely grown cultivars (Bayles et al , 1989; Boyd, 2005) and favourable weather (Gair et al , 1987; Johnson, 1992). Fungicides, particularly azole products, provide effective control of yellow rust when applications are well timed to protect the upper leaves (Cook et al , 1991, 1999; Paveley & Lockley, 1993; Paveley et al , 2000; Young & Paveley, 2003). Over 95% of winter wheat crops now receive fungicide products with activity against yellow rust, and this has contributed to its low severity in recent years (Hardwick et al , 2001).…”

Role of Fungicide Applications on the Integrated Management of Wheat Stripe Rust

“…The number of sporulant uredinia in the lower leaves are a measure of importance of the transfer of inoculum toward the upper leaves. Thus, for example, Young et al (2003) developed in England a model of prediction of the growth of the disease in a field plot based exclusively on lower leaf infections. These observations are in line with the assessments of Braithwaite et al (1998), who indicate that the time of SR onset is the most critical factor in defining the time of fungicide application, rather than the phenological stage per se.…”

A foliar disease model for use in wheat disease management decision support systems

“…Infections develop slowly over the winter and can be subject to loss due to frosts. Hence, the level of inoculum expressed in the spring can be an important risk factor (Young et al , 2003). Inoculum generated on the lower leaves is assumed to be the predominant source of infection of the upper leaves, so the source of inoculum consists of infectious lesions in the canopy.…”