After the last two years of high Stripe Rust pressure in the winter wheat crop, a common question throughout the spring is; How is the weather affecting Rust pressure? Here is the not so good news.
We observed a fair amount of rust in our wheat fields late last fall. The infections started from volunteer wheat and the high amount of inoculum in the fields after last years harvest. After infection of the new planted wheat, the pathogen grows within the leaf. Pathogen growth is most rapid at 41 to 51°F (current conditions), almost no growth is observed at temperatures below 25°F or above 85°F. If temperatures are outside the range for growth for any part of the day, the rust stops growing for that time but resumes growth when the temperatures become favourable again at other times of the day.
Sporulation of the fungal pathogen grows for about 14 days (shorter in some highly susceptible varieties, such as, Goetze) before the pustules erupt through the leaf. Spore production is favoured by high humidity (current conditions). Thus, fresh spores are typically seen in the morning because of cooler temperature and still air are more conducive for sporulation.
During high humidity in winter, most spores remain in small clumps: these are relatively heavy and fall out of the air quickly, so their spread is mostly over very short distances, leading to the ‘hot-spots’ of infection seen in crops in late winter and early spring. When hot spots are observed in a field, decreased yield has already occurred.
This post has not been created to make us feel even worse about the weather. However, even though we may be complaining about the poor conditions, Stripe Rust is loving it.