SPECIALTY CUT FLOWER CORNER: For the beginning grower

Flower diseases
Don't worry, be observant.

By Melanie DeVault

When it comes to flower diseases, just remember a couple of old sayings: Sanity rules (sounds good whether it’s old or not), and "an ounce of prevention...” you know that one. For the beginning flower grower, or the seasoned market gardener, the preventive stuff is the best way to keep disease at bay. That means working with your soil to keep it healthy, using compost and cover crops to add fertility and refresh the beds, using proper irrigation, rotating crops, and selecting disease-resistant varieties, especially for those flowers that can be a problem in your area. Trying to keep a little ahead of the weeds can help, too, although I’ve yet to meet a flower grower who doesn’t wind up apologizing to visitors for weeds. (Especially me.)

We’ve had very few problems with disease on flowers at Pheasant Hill Farm. We rarely even need organically approved disease fighters. My flower partner and I are always checking, and if something is fading or appears ill, it gets removed. Our main problem has been powdery mildew on monarda and zinnias. Last year, we had pretty good success using a milk spray (1 part whole milk to 9 parts water) to ward off powdery mildew on the zinnias. This year, we’ll start using the spray earlier, as a preventative. Since the leaves are stripped anyway and that’s where the problem hits, the flowers are saleable if the infestation isn’t severe.

About half of our China asters got hit with aster yellows last year, and this year we’re going to keep them covered with hoops and Reemay to try to keep the leafhoppers from turning them to mush. (I have to say we had more than enough flowers from the healthy plants last year.) We also had some rust problems in August in our biggest high tunnel. Damage was done to the leaves of agastache, dahlias and blue horizon ageratum. The lisianthus were unaffected. This year, we’ll try test spraying some peroxide to see if it helps.

So we don’t consider those little problems -- with the thousands of healthy flowers we sold -- real problems at all. We’ll keep working on simple solutions. Those mysterious disease problems will hit from time to time. You don’t have to be a professor of horticulture to get through them. There are many good books with pictures of diseases and detailed descriptions of their symptoms and possible solutions. The internet has tons of information. If you’re still not sure about a particular problem, contact or visit your local Extension office.

All in all, says Steve Bogash, regional commercial horticulture educator, Franklin County Cooperative Extension in Chambersburg, PA, diseases on cut flowers are low on the problem scale. He says it’s important to be observant, to catch any blips before they become problems.

“Powdery mildew is one of the most irritating problems, especially with zinnias, even with succession planting,” says Bogash. But, he adds, if you use a reasonable crop rotation, the small grower with diversified crops just shouldn’t have that many problems with disease.

Dr. Stanton Gill, with the Central Maryland Research and Education Center, Maryland Cooperative Extension, agrees. In addition to powdery mildew, bacterial leaf spot can also hit zinnias. But both men pointed out that these, and other fungal diseases often don’t kill the flowers. Just strip the leaves away and you can still sell those flowers in bouquets or bunches.

Watering Matters!

Something as simple as watering the right way can help you keep many diseases away, or from becoming serious problems. Using drip irrigation, or watering plants at the base instead of sprinkling or watering overhead can help you avoid botritis, alternaria and bacterial and other leaf spots, powdery mildew and anthracnose on flowers that are more susceptible to those diseases -- like zinnias and snapdragons. (More on those diseases follows).

Keeping insects under control is important in disease control, too, because they can spread viruses. And maintaining good air circulation can help avoid some problems like powdery mildew and botritis.

Some Common Diseases

Alternaria -- A fungal blight where leaves develop brown to black spots which enlarge and develop concentric rings like a target. Lower leaves are usually hit first. Spores are carried by air currents. Plant resistant varieties, rotate crops or use a disinfecting solution before planting (see Bacterial leaf spot below).

Anthracnose -- A fungal disease that causes small seemingly dead spots with sunken centers and raised borders. To control, rotate crops, buy disease-free seed and don’t touch plants when they’re wet.

Bacterial leaf spot -- Like alternaria causes spots on the leaves, but there is usually a yellow spot around it. Along with powdery mildew, this hits zinnias. Some growers recommend soaking seed in a mixture of 1 part bleach to 4 parts hot water (100 degrees), soaking for 20 minutes. They swear by it. Also, overhead watering encourages the problem.

Botrytis --A plant pathogenic fungus and it’s everywhere you grow plants. The disease it causes are gray mold or Botrytis blight. It hits many types of plants. Botrytis starts out looking like a white growth and turns gray as spores form. The spores are spread by wind or water. Botrytis needs a food source or nutrients to invade a plant, and this can come from old flower petals. With this food, the fungus gets aggressive and heads for healthy tissue. High humidity or moist conditions encourage growth of the fungus which is why greenhouses are a favored spot. To control it, keep things clean. Remove dying or dead leaves from plants and the soil surface and get it out of the greenhouse or away from outdoor plants. It’s a concern with lilies grown in crates. You’ll see it on the leaves, white and spreading out round. Feeding drip irrigation through crates to keep foliage drier and providing adequate ventilation help. Prevention is easier than dealing with the problem, says one long-time grower, who recommends trying a Bordeaux mixture (a combination of hydrated lime and copper sulfate) or foliar compost tea for problems. Organic growers needs to check the list of allowable materials, of course.

Damping-off -- Caused by fungi in the soil. The seedling looks like it was burned at the root level and die or seeds may rot before they germinate. Excessive moisture, those days of no sun in spring, and overcrowding seed flats encourage the disease, as does low soil temperature before germination. To avoid problems, use good sanitation practices -- disinfect trays, keep greenhouse benches clean, etc. -- and use bottom heat to until germination. We sprinkle milled sphagnum moss lightly across the top of flats if overcast days linger. Remove any plants from the cell that appear diseased, and if the entire flat appears problematic, get it out of the greenhouse.

Powdery Mildew -- Miserable fungi that love many flowers, including zinnia, monarda, phlox and several others. Mildew makes a white to grayish powdery growth on leaves. Then black dots appear and produce spores that the wind carries to other plants. It’s best to use preventive measures before there is a problem, with either the milk spray mentioned above, or a baking soda spray -- dissolve 1 teaspoon of baking soda and a few drops of soap in 2 quarts of water and spray on plants. Remove any infected plants.

© DeVault Enterprises 2005