I think most people who deal with plants understand insects. Simplified greatly: an egg is laid, a larva hatches, the larva eats the plants, the larva goes into a pupa phase (cocoon stage) and depending upon the type of insect, the adult may or may not eat plants again. Catch an insect problem in time and you can still rely on the plant to produce.
Diseases such as fungi and bacteria are different, however. First of all, we cannot see them when an outbreak is beginning, so the threat can go unnoticed. Diseases are microscopic. Second, the large majority of diseases of plants require moisture to be active. Moisture can come in many forms, not just rain: high humidity, heavy dews, fog or mists, irrigation, cloudy and overcast days. Moisture is a part of our daily life in the Midwest.
A third way diseases and insects are different is that spraying a plant with a disease rarely controls the disease or benefits that plant. In other words, disease control is preventative, not reactive as insect control is.
How do we control plant disease if we cannot react to it effectively? First of all, we try to anticipate conditions that cause the disease and put preventative treatments on before symptoms begin to occur. Apple growers have been doing this for decades for apple scab in the Midwestern fruit belt. Timing, they say, is everything. Second, we rely on plant breeders to develop varieties that are resistant or tolerant to the most devastating diseases.
We can also try to reduce the amount of inoculum (spores that can start the disease, for instance) by using crop rotation and sanitation practices. When I tell gardeners that commercial potato growers do not plant potatoes back into a field for five years or more to prevent disease, they are often shocked. For a small garden, crop rotation generally is not as useful as you would think, because we tend to use tillage equipment and irrigate, which can spread disease to parts of the garden where there were no potatoes.
Sanitation practices include removing sick plants or portions of plants as they are noticed and destroying those parts, as well as sterilizing tools used to prune plants. Farmers used to mold board plow fields for disease control to bury sickly crop residues. Gardeners often clean their gardens up at years end as a way to reduce potential problems the next season.
Burning or burying those sickly plants is generally recommended over composting. For composting to work for disease control purposes, the temperature of the pile will need to be maintained at more than 160 degrees for three or more days, which is difficult for most of us to do.
I personally learned the value of sanitation a few weeks ago. I have not planted impatiens at my home for more than 10 years, but this year, I created two new beds for this shade-loving flower. On Aug. 21, I noticed the larger of the two beds had a spot in it, which I attributed to an animal laying down in the bed overnight. By that afternoon, 70 percent of the flowers in that bed had lost all their leaves. Downy mildew had struck hard.
I cleaned the diseased bed very thoroughly and began to observe the other bed closely. As reports poured in from across the region that impatiens were going down, that bed remained healthy for more than two weeks.
I got the crazy idea to try to infect my healthy flower bed with downy mildew, to see how long it might take for the disease to show up. I scrounged up one infected leaf that was partially buried in the old bed and threw it into the middle of remaining healthy impatiens. In just two days, the patch was severely infected; in three days, every plant was dead.
The take-away message for me is that sanitation, while important, is probably more difficult than it appears, because only a little inoculum is needed to start an outbreak. No matter how hard I tried to clean up the old bed, there was still something remaining that could reinfect other plants. It will be years before I will be planting impatiens on our property.
Jeff Burbrink is a Purdue Extension educator in Elkhart County. He can be reached at 533-0554 or at firstname.lastname@example.org.