Spots and Rots and Blights...Oh My! Plant Diseases 101

You might find lions and tigers and bears frightening but farmers and gardeners would disagree. Groundhogs and rabbits and deer can be more destructive. And then there are the fungi. Early blight, late blight and anthracnose. Black spot and mildew and scab. Rusts and rots and wilts. It’s a jungle out there!

Fungi are certainly the most common organisms associated with plant disease. I say “associated” because the pathogen, by itself, does not equal disease. A susceptible host plant and the proper environment are also necessary.

Here’s a timely example. By now, tomato growers are probably noticing brown spots and perhaps yellowing of the lower leaves. On closer inspection, you may see that the “spot” is actually a lesion that has concentric rings of dead tissue. There may be tiny black dots in the dead tissue. As the season progresses more and more foliage is killed. Fruit infections cause soft spots and rots. All of these symptoms describe the common tomato disease called early blight. The disease also affects potato and eggplant. Some of you will be happy to learn that two weeds, horse nettle and black nightshade, are vulnerable, too. Since all of these plants are related, it is not surprising that they are susceptible hosts for the same pathogen. Note also that the early blight pathogen has no effect on asparagus, beans, cucumbers or you. (Sometimes folks ask if it is OK to eat diseased vegetables. It is.). The pathogen has a scientific name Alternaria solani.

OK we have a susceptible plant and a pathogen. The final ingredient needed for disease is the proper environment. This pathogen thrives and reproduces well under warm, moist conditions. The weather in the month of June, 2009 in Southeastern, PA was about perfect for early blight. Especially for tomatoes that were left to sprawl on the ground rather than trained to a support system which favored air movement and leaf drying.

Bingo! Early Blight of Tomato!

Maybe that was more than you wanted to know about early blight but it is useful. You can manage this disease (and all others) by thinking about three key disease ingredients: pathogen, host and environment. Sometimes it is possible and effective to eliminate the pathogen. But it can be equally effective to grow non-susceptible host plants or modify the environment. Disease resistant varieties have been developed for many of your favorite plants. Look for them when buying seed. Learn about the conditions that favor disease. Do what you can to change it. For most fungal diseases that means increasing air movement and reducing leaf wetness.

For a descriptions and gory pictures of common vegetable diseases, type “vegetable disease” into the search box at www.agsci.psu.edu.

For a complete guide to growing tomatoes and info on more tomato diseases go to http://pubs.cas.psu.edu/FreePubs/pdfs/uj230.pdf

Thanks to University of Minnesota Extension for the nice image of early blight shown above.