Gary C. Bergstrom, Plant Pathology and Plant-Microbe Biology Section, School of Integrative Plant Science, Cornell University
Wheat spindle streak mosaic (WSSM), caused by wheat spindle streak mosaic virus (WSSMV), is a disease that attracts little attention today because most of our widely grown, winter wheat varieties have significant levels of resistance to it. Yet WSSMV persists in New York soils in its protozoan vector ready to infect the roots of susceptible winter wheat varieties soon after planting. Swimming spores (zoospores) of the vector move through films of water in the soil and thus root infection is favored by moist conditions in fall. Plants remain infected over the winter dormant period but do not develop typical leaf symptoms until spring following a number of weeks of cool weather which favors virus replication and virus movement from roots into shoots. Temperature, not moisture, is what drives symptom development in spring since plants were already infected in the fall. Only winter wheats, not spring wheats, are affected by WSSMV because of the time it takes to build up virus levels in the roots and then the shoots.
Symptoms of WSSM first appear in late April or early May and are characterized by long, light green, spindle-shaped streaks with dark centers (Fig. 1). As leaves age, these streaks can become necrotic and resemble lesions of Septoria tritici blotch but without dark fruiting bodies, i.e., pycnidia of Zymoseptoria, in evidence under a hand lens. Symptoms of WSSM fail to develop on new leaves that emerge when average daily temperatures exceed 60 F, though symptoms can reinitiate at later growth stages if persistent cool conditions occur during stem elongation, head emergence, and even grain-filling. Conditions have been ideal in April and May 2020 for development of WSSM. Symptom development is extremely sensitive to warm temperatures such that we have seen very little WSSM in years with high temperatures in early spring.
What should a wheat producer do if she/he observes characteristic symptoms of WSSM this spring? There is no action that can be taken to mitigate WSSM in a growing crop – the yield damage, which can exceed 30% of the crop’s potential, has already occurred. However, diagnosis of the disease is a sure reminder that the variety they are currently growing is susceptible to WSSMV, and they need to choose a variety with at least moderate resistance for planting in the coming fall. This should elicit a conversation with your seed supplier about varieties resistant to WSSMV; some companies include that information on their website and in their seed catalogs but others do not. While the majority of available varieties express resistance to WSSMV, susceptible varieties appear in the seed market from time to time. Scores for WSSM also are included in winter wheat variety trial tabular results (https://blogs.cornell.edu/varietytrials/small-grains-wheat-oats-barley-triticale/small-grains-cultivar-trial-results/) from Cornell’s Small Grains Breeding Program in years when symptoms are observed.
If you find more pronounced mosaic and fewer distinct streak symptoms in a variety designated to be WSSMV-resistant, your wheat could be infected by another soilborne, protozoan-transmitted virus called soilborne wheat mosaic virus (SBWMV), which we have diagnosed occasionally in isolated fields in southern areas of the Finger Lakes Region. Resistance to SBWMV is independent from resistance to WSSMV, though it is also available from wheat seed suppliers in a choice of adapted varieties.