Plant Health Problems
See Perennials for a detailed discussion of problems that may occur and are common to most herbaceous ornamentals.
Diseases caused by Fungi:
Powdery mildew, Erysiphe cichoracearum.
This is by far the most common disease of phlox in Connecticut. Most cultivars are very susceptible to this fungus which appears as white, mealy growth on both surfaces of leaves and stems. Symptoms often first appear on the upper surfaces of the leaves and in the lower portion of the plant and are usually most pronounced during hot, humid weather. Heavily infected leaves turn brown and shrivel.
Disease can be minimized by avoiding overcrowded spacing of plants and by carefully picking off affected leaves as soon as symptoms are evident. Symptomatic leaves can be placed into a plastic bag in order to avoid spreading the spores of the fungus to other plants. Resistant varieties are also now available. Use of fungicides is usually not necessary but is helpful for highly susceptible cultivars. Applications can be made as soon as symptoms are visible. Among the compounds registered for use in Connecticut are horticultural oil, sulfur, potassium bicarbonate, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Leaf spot, Septoria divaricata.
Symptoms develop as distinctly circular, dark brown spots with white to gray centers. Infection usually starts on the lower leaves and progresses up the stem. Infected leaves curl and dry up.
Efforts to maximize plant vigor by fertilizing and watering are helpful. However, watering should be done early in the day to give the foliage a chance to dry before nighttime. It is also helpful to pick and remove symptomatic leaves as soon as they develop. Although not usually necessary, applications of fungicides can be made when new growth emerges in the spring. Among the compounds registered for use in Connecticut are mancozeb and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Diseases caused by Nematodes:
Bulb and stem nematodes, Ditylenchus dipsaci.
Infection by these plant-parasitic worms results in significant stunting of plants. Stems are frequently swollen or cracked and leaves are heavily deformed and appear thread-like or curled. Because of the presence of the nematode, many basal buds are stimulated to develop and plants have an unusual growth habit. In most cases, no flowers are produced and plants die prematurely. The nematode which causes this disease is systemic within the plant and comes from infested plants or soil and can be carried by water, garden tools, feet of animals, or in the seed.
Prevention is the most successful means for managing this disease. Phlox should not be planted in areas of known infestation. Symptomatic plants should be rogued and removed as soon as they are noticed.
Diseases caused by Physiological/Environmental Factors:
Leaf blight, physiological.
Symptoms first appear on the lower leaves as they shrivel, brown, and die. This disease is associated with old clumps and is rarely observed on seedlings or new transplants. Cuttings taken from symptomatic clumps are not affected nor is new growth arising from the crown or from the base of old stems. No causal agent has been identified but the problem appears to be the result of water imbalance in the plant as growth begins in spring--water is directed to the new shoots at the expense of the other ones.
Phlox plant bug, Lopidea davisi.
This bug injures phlox by puncturing the tender shoot or leaves at the growing tip and sucking the sap. It appears to be of minor importance in Connecticut. Should they become abundant, some of the compounds registered for control of this pest in Connecticut are acephate, azadirachtin or insecticidal soap foliar sprays, or imidacloprid used as a systemic to be taken up by the roots. Consult the labels for dosage rates and safety precautions.
Twospotted spider mite, Tetranychus urticae.
This is the most serious of all phlox pests. It infests the undersides of the leaves, which become light yellow in color and the plants have a generally unhealthy appearance. Sometimes the mites form webs, which more or less enclose the upper as well the lower leaf surface. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and ultrafine horticultural oil. Spraying with insecticidal soap will give sufficient control if applied at least twice at 7-10 day intervals. The predatory mite, Neoseiulus fallacis, is most commonly found feeding where there are mite infestations. A single application of ultrafine horticultural oil (1/2 - 1% dilution) can be effective if predatory mites are present. Special care should be taken with soap or oil to obtain thorough spray coverage, because they only work on contact. Hexythiazox or abamectin (a restricted use product) are choices appropriate for commercial growers. Consult the labels for dosage rates and safety precautions. Avoid applying carbaryl or pyrethroids, which tend to be much more toxic to the predators than to the pest spider mites.
Stalk borer, Papaipema nebris.
This borer occasionally tunnels in the stems of phlox and many other kinds of herbaceous plants. As a rule its presence escapes notice until the plant begins to wilt. Then it is too late for the plant to recover. The larva tunnels up and down inside the stem and the top portion usually wilts and later dies. There is one annual generation. The moths emerge in September and October and lay eggs on the stalks of their food plants, in which stage the insect passes the winter. The eggs hatch in May or early June. The young larva begins to feed on the leaves of the nearest food plant, and later tunnels in the stem. The mature larva is nearly 1 ½" long, grayish brown with one white dorsal stripe and two white lateral stripes on each side. On the front half of the body the lateral stripes are interrupted, and the lower brown stripe extends forward onto the side of the head.
Burning all the old stalks and destroying weeds at the edges of the garden, if allowed, helps control this insect. When needed, methoxychlor, which is among the compounds registered for use against this pest in Connecticut, applied as a dust, in June, should control this pest. Consult the label for dosage rates and safety precautions.