From Clemson Plant Pathologist Tony Keinath.
Plants are a rich source of food for microorganisms—for aggressive plant pathogens, for weak pathogens, and for common saprophytes (the 90% of microorganisms that feed only on dead plant parts). Once a pathogen kills parts of leaves or side roots and we see dark brown spots, things change. The normal ways plants defend themselves from pathogens shut down, and it’s a “free for all” for any microorganisms on the leaf, fruit, or root. These weak pathogens and saprophytes suddenly can grow on parts of the plant that weren’t accessible to them when the plant was healthy. It’s part of the natural succession of the phytobiome, the community of microorganisms that live in, on, and around plants.
For a diagnostician, it’s infinitely easier to find a pathogen at the early stages of disease, when suspicious yellow spots appear on a leaf, rather than on a dead leaf, because you’re not “looking for a needle in a haystack,” or more accurately, a pathogen in a forest of microorganisms. Cucurbit leaves, for example, tend to have the same three saprophytes on them, Alternaria, Epicoccum, and even Fusarium. An experienced diagnostician knows to ignore them or use them as forensic clues that the tissue has been dead for some time. I tend to suspect spray burn when I see these fungi growing in distinct spots on leaves.
Every diseased plant sample collected for diagnosis, whether it is for in-person delivery or mailing, should be carefully selected and handled by following these guidelines. Samples collected during the summer or on sunny days at any time of the year should be placed in a cooler with an ice pack after collection. This usually means taking the cooler into the field. Proper handling of samples will help the diagnostician provide an accurate answer or avoid asking for another (better) sample.
As another saying goes, “early is on time.” An early diagnosis is a timely diagnosis.