The Fungus Among Us

We started the gardening season with dry conditions. For plant diseases, dry is not a good word, because most diseases that occur in the landscape need moisture in the form of humidity or liquid water. But then in June it rained and then rained some more. And for the past two weeks it’s been soupy at best, and plant disease pressure is high. Then, quite predictably, the disease calls began coming in. Golf courses began to show turf disease issues. Growers began to report a plethora of diseases in roses and then the fruits, the vegetables and now our ornamentals.The streaks have already shown up on the delphinium foliage, and the powdery white stuff (mildew) is appearing on the foliage of the roses, lilacs, phlox and several garden veggies like zucchinis. Rose foliage may be dropping as if it were November. Wonderfully swollen buds and flowers on roses, poppies and peonies suddenly got fuzzy, then mushy and fell to the ground. Small black spots will show up on the tomato leaves, and things mysteriously begin to rot in the ground.

All of these problems fall into an amorphous area that we simply call plant diseases, and they include mildews, rusts, wilts, leaf spots, cankers, rots and a host of viruses. Unfortunately, the study of these problems is a very complex science and only in the past 45 years have plant scientists (pathologists) been able to develop species and varieties that resist or are immune to these problems. However, their work is slow and very time-consuming. In the meantime, we can learn to identify as many of these problems as possible and how to cope with them. As you will see, poisons are not always the answer.

Fungi can be microscopic plants that contain no chlorophyll and reproduce by spores that are usually spread by mycelium. In general terms, fungi are the largest cause of plant disease.

Bacteria are microscopic organisms that enter a plant through natural openings or injuries and cause infection. Bacterial diseases include soft rots, galls, scabs, blights and vascular wilts.

Viruses are tiny specs of nucleic acid and protein that multiply only on living cells and have the ability to cause disease in the plant. They cannot penetrate directly, but must enter through a wound or are injected by a feeding insect such as a nematode or an aphid. Common virus diseases include the mosaics, stunting, leaf spot and ring spot.

There is also another kid on the block called MLO (mycoplasma-like organism) that is associated with nearly 50 plant diseases of the “yellows” type. Viruses are thought to be the causal agent and the viruses are thought to be transmitted by leaf-hoppers.

Anthracnose as a fungal disease can appear on snap beans, limas and peas. The problem appears as black, sunken lesions on pods that exude pinkish ooze in moist weather. Leaves and stems may develop small black spots, and veins on the underside of the foliage turn black as well. Cool, wet weather encourages the disease, and working around wet plants may hasten the infection. Rotate crops and destroy crop residues, or spray with an organic or chemical fungicide as per your convictions. Anthracnose occurs also on watermelons, cucumbers and muskmelons, but in a different form. Symptoms begin as small, yellow, water-soaked spots on the leaves that rapidly enlarge, turn brown (or black on watermelons), and break through, causing a jagged hole within the spot. Fruit symptoms are similar: dark, sunken lesions, which enlarge rapidly. In moist weather pink ooze exudes from the center of the spot and humid, rainy weather hastens development. Just to confuse matters, anthracnose also occurs in trees and on lawns, with differing symptoms and resolutions. The Hamptons were once dotted with a weedy tree called the silk tree or mimosa tree, but they’ve all but disappeared because of an anthracnose attack over 35 years ago.

Downy mildew is a fungus disease that affects grapes, cucumbers, onions, lettuce, melons and crucifers. The symptoms are yellow, angular-shaped areas of infected tissue on the upper surface of the leaves. Underside, the leaf may appear white to purplish “downy” in color, especially in wet, humid weather. The best protection is to plant and grow resistant varieties or spray a protective fungicide as per the label instructions.

Powdery mildew is yet another fungus disease that affects many plants, including squash, cucumbers, phlox, roses, lilacs and more. And no, the mildew on your squash can’t spread to your lilac. Same disease, but different species of the disease. Mildews are host-specific, affecting only one type of plant. The symptoms are easy to recognize as white, powdery growth on the upper surface of the leaves and stems. There are resistant varieties of both veggies and ornamentals (as in mildew-resistant types of Monarda and phlox varieties such as David), and some extremely effective chemical and organic sprays are available, but treatments need to start early, before the symptoms appear. Treatment once the disease is present won’t make it disappear where the infection has taken place, but it will stop the infection from spreading. It should also be noted that powdery mildew, although unsightly, has probably never killed a lilac.

Yellows or mycoplasmas may occur on lettuce, carrots, asters and onions. As the name implies, yellowing of the leaves is a symptom, as well as dwarfing, twisting and curling of the foliage. In lettuce, premature bolting may occur, and with carrots you may notice extremely hairy taproots. The disease is transmitted by insects and thus control is through elimination of the vector (transmitting insect) with an appropriate insecticide early in the season.

Ring spot of peppers is a virus disease that affects tobacco and flowers and fruits as well. Stunting and mosaic on the foliage is obvious if the infection takes place early in the season, while later infection usually results in ring-spotting of the fruits and leaves. Again, insects are the culprits as aphids are the vectors. So far there are not any resistant varieties of plants, and the only control is to remove infected plants and control the aphids earlier in the season.

Bacterial wilt is spread by cucumber beetles and can show up not only on cukes but also on melons and squash. The symptoms usually show up as a wilting and drying of the leaves, runners or the entire plant. Injury from the feeding beetle is usually present and a good clue that danger lurks. A simple test for this wilt is to cut a wilted stem near the crown, and touch a clean knife to the cut stem, and if threads of white ooze stretch out between the knife and stem, you’ve got it.

Again the control is to eliminate the insect early in the season, which means knowing when to look for the beetles and where to look for them as well. The bacteria overwinter in the beetles and are injected into the plants during feeding. Usually, early applications of an appropriate control will curb this bug.

Well, the list is only half done and next week we’ll finish it off. Hopefully you have noticed a not-too-subtle relationship between insects and plant diseases and you may understand why I harp on insect control early on in the planting and growing seasons. As in weeds, insects controlled as soon as they appear dramatically reduce or eliminate many related problems that may not show up until weeks after the pest has traveled on. Also next week, a great and easy way to diagnose your tomato diseases. Keep growing.