By Andi Anderson
Aster yellows is a plant disease that affects crops like carrots and celery. It is spread by a small insect called the aster leafhopper. Each year in Michigan, these insects begin appearing in May, bringing the risk of infection to farms.
The main concern for farmers is not just the number of leafhoppers, but how many of them are carrying the disease. This is known as infectivity. Studies from the 2024 and 2025 growing seasons show that infectivity changes throughout the season, making timing very important for disease control.
In both years, the level of infectivity was low at the start of the season in May. As the season progressed, infectivity increased during mid-summer and then declined later. However, the exact timing and intensity were different each year. In 2024, infectivity rose in mid-June, while in 2025, the highest levels were seen from late June to early July. The 2025 season also showed more sudden changes in infectivity from week to week.
Weather conditions, such as warmer temperatures, supported insect activity but did not fully explain the differences between the two years. Wind patterns also played a role, especially in May, when westerly winds helped keep infectivity levels low.
The study also highlighted differences between crops. In carrot fields, infectivity increased quickly for short periods, creating sudden high-risk situations. In celery fields, infectivity remained lower but lasted longer, leading to extended periods of moderate risk.
These findings show that farmers should focus on timing rather than average infection levels. Short periods of high infectivity can cause the most damage, especially during mid-summer when the risk is highest.
Different crops require different management approaches. Carrot producers need to act quickly when risk levels rise, while celery growers should maintain consistent monitoring and control methods.
Although the data is based on only two years, it provides useful early insights. With continued monitoring, farmers can better understand disease patterns and protect their crops more effectively.
Photo Credit: gettyimages-dmaroscar
Categories: Michigan, Crops