Diagnosing plant problems is a learnable skill that gets easier with experience. Making a proper diagnosis may require the help of a university or private laboratory, but growers can often combine their background knowledge with careful observations to make a good working diagnosis. Here are some tips for diagnosing your pest problems in the greenhouse.
1. Look closely at individual plants, noting which leaves or other parts exhibit symptoms.
2. Use a magnifying tool to look for signs of the presence of arthropods (the pests themselves, eggs, cast skins, frass, etc.), mold, and fungal spores on the top and bottom leaf surfaces.
3. Remove a few plants from their containers. Look for poorly developed root systems, scorched tips, soft decay, discoloration, or evidence of feeding on roots or root hairs.
4. Don’t forget the big picture. Do symptoms occur most often near doors, bench edges, where the roof drips, or where flood floors drain most slowly? If the answer to any of the above is “yes,” cleaning up standing water might be a good start toward containing or even solving the problem.
5. Consider the history of the crop and your operation’s cultural practices. Have any materials been used on this crop for the first time, or have you recently used a familiar material at a higher rate than usual?
6. Consult reference books, Extension literature, and notes from grower seminars. Talk with experienced growers, Extension agents, and consultants.
7. Form a tentative diagnosis, and send samples representative of early and intermediate stages of the problem to a diagnostic lab.
8. After determining the problem, discard plants with contagious diseases and take appropriate control measures.
9. Continue to watch the crop carefully. The initial diagnosis may only be telling part of the story. For example: You found the fungus gnats and treated the crop for them, but the Pythium infection they were carrying might not show up until later.