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
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
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
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
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
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.