Performance Story: Aster yellow disease in spring wheat
In 2012, a disease called aster yellows (AY), which is caused by a phytoplasma vectored (transferred) among plants primarily by aster leafhoppers reduced yields in 80% of canola fields across Saskatchewan. Wheat crops also suffered an unexpected yield loss that year without any apparent cause. The only potential clues were reports of the presence of white wheat heads in many fields with this phantom yield loss. What could have reduced yield in wheat that year? Could it have been infection of wheat with the phytoplasma that caused aster yellows in the canola crop? But, you might ask, canola is the only crop where insecticides are registered to control aster leafhoppers, isn’t it? While that assessment certainly is true, aster leafhoppers actually prefer cereal crops as food and reproductive hosts over canola but canola is susceptible to infection by phytoplasmas and readily expresses aster yellows symptoms such as “bladder-like” pods, prolific branching of stems, leaves where there should be flowers and leaves where there should be seeds in malformed or seemingly-normal pods. Manuscripts from the 60s and 70s describes specific symptoms of aster yellow such as stunting, excessive tillering, malformed leaves, and white, sterile heads, on artificially inoculated barley and wheat. More recent manuscripts describing similar symptoms in fields suggested that most cereal plants infected with aster yellows exhibit no symptoms and that this disease might be common but largely undetected due to low symptom expression. These results from the literature agree with our experiments.
This project sought to duplicate the phantom yield loss of wheat experienced in 2012, attempted to generate symptoms of aster yellows in popular wheat and durum cultivars and assess attraction of popular wheat cultivars to the AY vector, aster leafhoppers. In the field, aster leafhopper populations were monitored in wheat and durum varieties to discover if any varieties were more attractive or perhaps led to increased offspring production and also to evaluate each variety for the presence of aster yellows symptoms and phytoplasma. Other questions that we had were how susceptible wheat and durum plants are to aster yellows, how often do we see any of the potential AY symptoms in wheat exposed to infected aster leafhoppers, and what environmental conditions might lead to increased expression of symptoms.
Aster leafhopper field populations were not at outbreak levels during the project years like they were in 2012 but leafhoppers were present and pressure on our experimental field plots was consistent over the experimental years. Even with leafhoppers, some infected with aster yellows, present in the field plots, none of the potential symptoms of aster yellows were obvious in the plots. We quickly realized that any of the aster yellows symptoms in wheat could easily be confused with symptoms of other infections such as fusarium and that definitive proof of aster yellows infection in wheat could only be found through molecular diagnostic tests. So, we optimized these molecular diagnostic tests and adapted them for use in wheat. We demonstrated that detecting aster yellows with molecular tests in an infected plant is twice as likely by sampling wheat stems and was least detectable when sampling leaf tissue.
Durum wheat attracted more adult leafhoppers into experimental plots than did spring wheat and in leafhopper infection experiments durum was more susceptible to infection and symptom expression than spring wheat. In field plot there were no trends towards any of the spring wheat varieties being more attractive to adult leafhoppers or producing more leafhopper offspring. We learned then that durum is more attractive to aster leafhoppers and showed more AY symptoms than spring wheat. In regard to environmental conditions, and, similar to canola, more AY symptoms were expressed in wheat under wet soil and high light conditions than under dry soil and lower light conditions. Our experiments mimicked the wet soil conditions of the spring followed by high light (sunlight is always high light!) of 2012 and led to lower yields than control plants without aster yellows infection. Wheat and durum plants were more likely to develop symptoms of aster yellows following infection at the three leaf stage than at the three tiller stage showing that early infection of plants has worse consequences than later infection plants. These results points to infection by aster yellows in 2012 as the smoking gun of the unexpected yield loss in wheat that year when aster leafhoppers arrived early and infected and soil conditions were wet.
Overall though, in both field and laboratory experiments, aster yellows symptoms were difficult to create in the experimental plants, even with the highest densities of leafhoppers tested and symptoms were not consistent even when plants were infected. We must conclude that, except for outbreak year with very high level of infection, aster yellows incidence in wheat will likely remain low in Western Canadian agriculture. We have also learned that if we need to diagnose aster yellows infection in wheat we now have reliable molecular tools.In 2012, a disease called aster yellows (AY), which is caused by a phytoplasma vectored (transferred) among plants primarily by aster leafhoppers reduced yields in 80% of canola fields across Saskatchewan. Wheat crops also suffered an unexpected yield loss that year without any apparent cause. The only potential clues were reports of the presence of white wheat heads in many fields with this phantom yield loss. What could have reduced yield in wheat that year? Could it have been infection of wheat with the phytoplasma that caused aster yellows in the canola crop? But, you might ask, canola is the only crop where insecticides are registered to control aster leafhoppers, isn’t it? While that assessment certainly is true, aster leafhoppers actually prefer cereal crops as food and reproductive hosts over canola but canola is susceptible to infection by phytoplasmas and readily expresses aster yellows symptoms such as “bladder-like” pods, prolific branching of stems, leaves where there should be flowers and leaves where there should be seeds in malformed or seemingly-normal pods. Manuscripts from the 60s and 70s describes specific symptoms of aster yellow such as stunting, excessive tillering, malformed leaves, and white, sterile heads, on artificially inoculated barley and wheat. More recent manuscripts describing similar symptoms in fields suggested that most cereal plants infected with aster yellows exhibit no symptoms and that this disease might be common but largely undetected due to low symptom expression. These results from the literature agree with our experiments.
This project sought to duplicate the phantom yield loss of wheat experienced in 2012, attempted to generate symptoms of aster yellows in popular wheat and durum cultivars and assess attraction of popular wheat cultivars to the AY vector, aster leafhoppers. In the field, aster leafhopper populations were monitored in wheat and durum varieties to discover if any varieties were more attractive or perhaps led to increased offspring production and also to evaluate each variety for the presence of aster yellows symptoms and phytoplasma. Other questions that we had were how susceptible wheat and durum plants are to aster yellows, how often do we see any of the potential AY symptoms in wheat exposed to infected aster leafhoppers, and what environmental conditions might lead to increased expression of symptoms.
Aster leafhopper field populations were not at outbreak levels during the project years like they were in 2012 but leafhoppers were present and pressure on our experimental field plots was consistent over the experimental years. Even with leafhoppers, some infected with aster yellows, present in the field plots, none of the potential symptoms of aster yellows were obvious in the plots. We quickly realized that any of the aster yellows symptoms in wheat could easily be confused with symptoms of other infections such as fusarium and that definitive proof of aster yellows infection in wheat could only be found through molecular diagnostic tests. So, we optimized these molecular diagnostic tests and adapted them for use in wheat. We demonstrated that detecting aster yellows with molecular tests in an infected plant is twice as likely by sampling wheat stems and was least detectable when sampling leaf tissue.
Durum wheat attracted more adult leafhoppers into experimental plots than did spring wheat and in leafhopper infection experiments durum was more susceptible to infection and symptom expression than spring wheat. In field plot there were no trends towards any of the spring wheat varieties being more attractive to adult leafhoppers or producing more leafhopper offspring. We learned then that durum is more attractive to aster leafhoppers and showed more AY symptoms than spring wheat. In regard to environmental conditions, and, similar to canola, more AY symptoms were expressed in wheat under wet soil and high light conditions than under dry soil and lower light conditions. Our experiments mimicked the wet soil conditions of the spring followed by high light (sunlight is always high light!) of 2012 and led to lower yields than control plants without aster yellows infection. Wheat and durum plants were more likely to develop symptoms of aster yellows following infection at the three leaf stage than at the three tiller stage showing that early infection of plants has worse consequences than later infection plants. These results points to infection by aster yellows in 2012 as the smoking gun of the unexpected yield loss in wheat that year when aster leafhoppers arrived early and infected and soil conditions were wet.
Overall though, in both field and laboratory experiments, aster yellows symptoms were difficult to create in the experimental plants, even with the highest densities of leafhoppers tested and symptoms were not consistent even when plants were infected. We must conclude that, except for outbreak year with very high level of infection, aster yellows incidence in wheat will likely remain low in Western Canadian agriculture. We have also learned that if we need to diagnose aster yellows infection in wheat we now have reliable molecular tools.