Impact of Meteorological Conditions and Maturity of Perithecia on the Release of Fusarium graminearum Ascospores

The global food supply is being stressed by climate change, a growing population, and harmful diseases. One risk to vital cereal crops such as wheat and barley is Fusarium head blight (FHB), caused by the fungal plant pathogen Fusarium graminearum. Ascospores of the fungus are released from perithecia on the residues of corn and small grains and can be transported long distances (>500 m) through the atmosphere. The overall objective of this work was to assess the influence of meteorological conditions and perithecial maturity on ascospore release. The research focuses on F. graminearum because of its damaging impact to staple crops and the global ubiquity of FHB.

The first specific objective was to apply state-of-the-science techniques to identify causal meteorological variables of ascospore release. We analyzed field measurements of airborne ascospores against meteorological conditions at Virginia Tech's Kentland Farm, Blacksburg, Virginia, USA and used convergent cross mapping and multivariate state space reconstruction to identify significant causal agents within this complicated natural and dynamic system. We identified relative humidity, solar radiation, wind speed, and air temperature as predictors of ascospore release.

Our second research objective was to understand the impact of varying meteorological conditions on ascospore release under controlled environmental conditions. We assessed ascospore release in a chamber with controlled temperature (15°C and 25°C) and relative humidity (60%, 75%, and 95%). Ascospores released from ascospore-producing structures (perithecia) were captured on microscope slides placed inside of 3D-printed ascospore discharge devices. Results showed the sensitivity of ascospore release to relative humidity and temperature, with cool temperature and high relative humidity resulting in greater quantities of ascospores released.

Our third research objective was to determine the relationship between the maturity, the number of ascospores, and the hardness of perithecia. A mechanical compression testing instrument was used to investigate the hardness of perithecia at various stages of maturity, producing a mean perithecium compression constant quantifying the uniaxial compression force required to rupture a perithecium. Results indicated that old perithecia contain the greatest amount of ascospores and exhibit increased resiliency, requiring greater forces to rupture, compared to young perithecia.

This research has illustrated the complexities of F. graminearum ascospore release by describing the impact of several meteorological conditions and perithecial maturity on the timing and quantity of released ascospores. Collectively, our results may inform wheat growers on the nature and timing of ascospore release, which could help inform FHB management decisions in the future.