Plant Health Problems
Diseases caused by Fungi:
Rhizoctonia crown rot, Rhizoctonia solani.
Infection first appears as a general decline, yellowing, and wilting of the entire plant. This is usually followed by complete collapse of the plant. Brown to black lesions can often be seen at the base of the stem, usually at the soil line.
Control can be difficult once plants are infected so prevention is important. It is helpful to avoid overwatering, especially in heavy soils, and to avoid watering directly into the crown area of the plant. Adequate spacing between the plants can promote good air circulation. Highly symptomatic plants can be rogued and removed since recovery is unlikely. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are iprodione and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Vascular wilt is a troublesome disease of impatiens, especially in beds that have been planted to impatiens for many years. Plants may be attacked at any stage; young plants can suddenly dry up or older plants might develop a pale green color accompanied by wilting of lower leaves, often first appearing on one side of the plant. This is frequently followed by a general wilting and death of the entire plant. Plants may wilt in the middle of the day and seem to recover at night. When the stem is cut, a black discoloration or streaking may appear in the vascular tissues.
Control of this disease is difficult since the pathogen is commonly found in soil. One of the key strategies for control of vascular wilts is prevention. Therefore, it is important to avoid planting in infested soil. It is also helpful to maximize plant vigor by good cultural care and watering. Careful handling of plants will avoid root injury which enables the fungus to enter the plant. Since repeated use of the same area greatly increases the amount of disease, rotation is essential. When available, it is also helpful to use resistant varieties. Chemical controls are not effective for these fungi.
Leaf spots, Alternaria, Cercospora.
Symptoms first appear as tan to brown to black spots with purple margins and vary with the causal fungus. Spots often appear on lower leaves first and can gradually spread up the plant.
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 chlorothalonil, thiophanate-methyl, iprodione, and sulfur. Consult the label for dosage rates and safety precautions.
Powdery mildew, Erysiphe.
White powdery spots or patches develop on leaves and occasionally on stems. Symptoms often first appear on the upper surfaces of the leaves 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. Use of fungicides is usually not necessary. However, applications can be made as soon as symptoms are visible. Among the compounds registered for use in Connecticut are horticultural oil, potassium bicarbonate, copper sulphate pentahydrate, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Botrytis blight, Botrytis cinerea.
Flowers turn a papery brown and become covered with gray, fuzzy masses. Senescing flowers are particularly susceptible. Tan to brown spots with a target-like appearance can also develop on the leaves. These patches are often associated with flowers which have dropped onto the leaf surface. This disease is particularly troublesome during periods of extended cloudy, humid, wet weather.
Good sanitation practices including grooming the plants and removing spent or senescing flowers can minimize the potential for infection. These affected tissues should be carefully removed and discarded when they are dry. It is also important to avoid wetting the flowers when watering and crowding plants. Adequate spacing between the plants can promote good air circulation. Use of fungicides is usually not necessary. However, control can enhanced with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are mancozeb, copper sulphate pentahydrate, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Diseases caused by Viruses:
Impatiens necrotic spot and Tomato spotted wilt, viruses (INSV and TSWV).
These diseases are the most important problems on impatiens and can be extremely damaging. They are particularly troublesome on double-flowering varieties. Symptoms can appear as black ringspots, mosaics, and necrotic spots with tan centers and brown margins. Petals can develop a color breaking and leaves can pucker, become distorted, yellow, and abscise. Stems occasionally develop distinctly black areas or lesions and plants can be stunted. These pathogens are transmitted by the western flower thrips.
Control of these diseases is focused on prevention since once plants are infected, they cannot be cured. It is important to eliminate and remove infected plants as soon as they are recognized and to eliminate other symptomatic plants since these viruses have very broad host ranges. It is also critical to manage the thrips population.
Diseases caused by Nematodes:
Root-knot nematodes, Meloidogyne spp.
Plants appear unthrifty, small, and weak. Above-ground symptoms are nonspecific as plants appear stunted, yellowed, and wilted. Diagnostic symptoms can be observed by the presence of galls on the roots. These can range in size from small swellings to large, distinctly clubby galls. The northern root-knot nematode, M. hapla, is a sedentary endoparasite, meaning that it infects host roots after hatching from eggs, stimulating the formation of a small gall containing specialized feeding cells, and feeds in the same location through several molts to produce several hundred offspring. Because most of its life cycle is inside roots, it may be spread to new locations with vegetative propagation material. This nematode is parthenogenetic, a single female can reproduce without males, resulting in a new generation every 28 days under ideal conditions. The galls produced on roots interrupt translocation and act as a nutrient sink. The nematode has a wide host range, but a number of ornamentals, including Rudbeckia, Aster, and others, have been shown to be resistant.
Control of this disease is very difficult since these nematodes have a wide host range. Growing resistant plants or rotating to small grains can greatly reduce or eliminate nematode populations in infested soil.
Diseases caused by Physiological/Environmental Factors:
Dodder, Cuscuta spp.
This parasitic seed plant is recognized by the presence of yellowish-orange strands which wrap around the plants like masses of spaghetti. Because dodder is a leafless plant which does not contain chlorophyll and therefore cannot manufacture its own food, it sends haustoria or sinkers into stems and leaves of the impatiens in order to obtain nutrients. Flower clusters are visible on the strands in midsummer.
Control of this plant parasite is extremely difficult. It is very important to rogue and remove any plants with symptoms as soon as possible after they are detected and especially before the dodder develops flowers. If dodder has been a problem in a particular planting bed, it is helpful to avoid planting in that area for several years.
Balsam aphid, Macrosiphum impatientis.
This aphid can be a problem on impatiens and balsam. Control is seldom needed.
Western flower thrips, Frankliniella occidentalis.
This insect can be an important problem in greenhouses because it transmits tomato spotted wilt virus. When needed, insecticidal soap, registered for use against this pest in Connecticut, can be used to suppress thrips populations. Control with organophosphate, carbamate, or pyrethroid insecticides may be difficult due to insecticide resistance. Options appropriate for commercial growers include abamectin (restricted use) or spinosad. Consult the label for dosage rates and safety precautions.