Night-time spore deposition of the fusarium head blight pathogen, Gibberella zeae, in rotational wheat fields

Many fungal plant pathogens rely on atmospheric motion systems for transport. The movement of these pathogens in the atmosphere is characterized by processes of liberation, drift, and deposition. We observed temporal patterns of viable spore deposition of Gibberella zeae, causal agent of fusarium head blight (FHB) of wheat, over rotational wheat fields (with no visible residues of corn or wheat, potential inoculum sources of G. zeae) in Aurora, New York, United States. Viable, airborne spores of G. zeae were collected on Petri plates containing selective medium placed 30 cm above wheat canopies. Over 40 000 viable spores of G. zeae were collected over a total of 73 day (6:00 am to 8:00 pm) or night (8:00 pm to 6:00 am) sampling periods in 3 years (2002, 2004, and 2005). The vast majority of the spores was collected at night (94% in 2002, 86% in 2004, and 82% in 2005). Viable spores were deposited in all but one sampling period spanning spike emergence through kernel milk stages of local wheat. Seven major deposition events (>50 colonies, on average, per Petri plate) were observed, all at night, and three of these were coincident with rainfall. In a single wheat field in 2004, viable spores of G. zeae were collected every 2 h throughout the night (starting at 8:00 pm) for six consecutive nights. Overall, 87% of the spores were deposited from 12:00 pm until 6:00 am, with most of the spores deposited between 4:00 am and 6:00 am. In 2005, viable spores of G. zeae were collected over variable landscape environments (two winter wheat fields, two soybean fields, one alfalfa hay field, and one fallow corn field) within 1 km2 area. Temporal patterns of viable spore deposition were virtually identical over all of the landscape environments, suggesting that spores were being deposited over kilometer distances from a well-mixed atmospheric source of inoculum. Atmospheric settling (gravitational fallout of spores) coinciding with the development of an inversion layer may explain the high degree of spore deposition at night. Vertical mixing and the presence of a mixed or turbulent layer during the daylight hours may account for the relatively low number of spores collected during the day. The deposition of spores of G. zeae primarily at night in rotational wheat fields raises some interesting questions about the origin of inoculum for epidemics of FHB. Our results suggest that spore deposition may be separated from spore release in both time and space. The cumulative exposure of wheat spikes to airborne spores of G. zeae, and the deposition of these spores mainly at night, should be considered in future prediction models and management strategies for fusarium head blight.