In NH we also have that pesky fungus (red thread). That too will cross contaminate from lawn to lawn (with a dirty mower). The only way to avoid it would be a thorough deck scrubbing and then spray deck and blades down with 10% bleach mixture with water to kill the fungus. Once you've got it, well, you have got it. I have chased it for years and now have just accepted it. You can treat it, but the treatment is expensive and only works for a few weeks.