A detached spear screen for purple spot (<i>Stemphyllium vesicarium</i>) on asparagus spears which correlates with natural field infection

An effective screening method for purple spot disease, caused by Stemphylium vesicarium, in asparagus spears is crucial for breeding resistant cultivars. Field evaluations from natural infection are often inconsistent due to environmental variability, and assays using inoculated cut spears have shown unreliable results. The objective of this research was to develop a growth chamber screening method that correlates with field disease severity and can be readily adopted by breeding programs. Intact spears of four asparagus cultivars, 'Gijnlim' (G.J.), 'Guelph Millennium' (G.M.), 'Guelph Eclipse' (G.E.), and 'Jersey Giant' (J.G.), were evaluated for disease severity during natural field infections. Spears were also harvested when field infections were low, wounded, inoculated with conidia of S. vesicarium, placed standing in solutions of 0%, 5%, or 10% sucrose, incubated for seven days under high humidity, and evaluated. Endogenous carbohydrates were also measured in spears without disease for separate field and growth chamber experiments. Under natural infections, G.J. and G.M. had lower disease severity and higher carbohydrate concentrations compared to G.E. and J.G. For cut, inoculated spears in the growth chamber, disease severity correlated with field infections only when spears were incubated with bases standing in 5% or 10% sucrose. Resistant cultivars showed stable carbohydrate levels, while susceptible cultivars exhibited carbohydrate accumulation that was associated with increased disease severity. The growth chamber assay provides a reliable and practical screening method for resistance to purple spot, offering a valuable tool for breeding improved disease resistance in asparagus.