Assessing the impact of abiotic conditions on fungal pathogen spread by onion thrips

Onion thrips (Thrips tabaci Lindeman) is a major insect pest of onion (Allium cepa) causing damage through feeding and contributing to disease spread. In Michigan, Colletotrichum coccodes (Wallr) is a fungal pathogen of onions, causing tan lesions on the foliage, and reducing yield. Onion thrips perform best on C. coccodes infected onions compared to uninfected plants, but it is unclear how increased performance is related to pathogen spread. To better understand this relationship, we released 5 or 25 thrips in arenas with one C. coccodes inoculated onion and 3 non-inoculated onions. Thrips were allowed to move freely among onion plants for 13 days. At the end of the experiment, we recorded thrips numbers, their location and estimated the damage on each plant. We recultured C. coccodes from some experimental plants by placing leaf tissue on a 25% strength potato dextrose agar. We found that higher thrips numbers lead to increased foliar damage but higher thrips numbers did not affect disease spread. To further investigate how environmental conditions may impact thrips and disease spread, we tested disease spread at two different temperatures (33℃ and 22℃) and two different relative humidities (~95% and ~35%). Five thrips were released in an arena containing 1 inoculated onion and 2 healthy onions and allowed to move freely among the plants for 14 days. At the end of the experiment, thrips location as well as the estimated percent damage on each plant was recorded.