Plant Health Problems
Diseases caused by Fungi:
Powdery mildew, Erysiphe, Oidium.
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.
Leaf spots, Phyllosticta, Gloeosporium, Cladosporium.
Circular to irregular tan to brown spots develop on leaves. These can vary in size, color, and number.
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, mancozeb, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Botrytis blight or blotch, 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 be enhanced with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are chlorothalonil, mancozeb, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Root rots, Thielaviopsis basicola, Pythium sp., Rhizoctonia solani.
The above-ground symptoms of root rots are non-specific and include a general wilting, decline, and collapse of the foliage and the entire plant. This general droopiness or flaccid appearance is often accompanied by browning and rotting of the roots and the crown. Yellowing and death of the outer leaves follows, until finally the entire plant is dead.
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. Highly symptomatic plants can be rogued and removed since recovery is unlikely.
Diseases caused by Bacteria:
Bacterial leaf spot, Xanthomonas begoniae.
Symptoms include small blister-like spots that appear on both tuberous and fibrous begonias. The spots are translucent at first and as they age, the centers dry to a tan color. The margins of the spots usually remain translucent or water-soaked. The bacteria may become systemic and cause collapse and death of the plants.
This disease can be minimized by improving air circulation by thinning the plants and by avoiding overhead irrigation. Picking and destroying infected leaves is also helpful. Chemical control is not usually necessary but can supplement other methods for disease management. Among the compounds registered for use in Connecticut are elemental copper, copper hydroxide, and copper sulphate pentahydrate. Consult the label for dosage rates and safety precautions.
Diseases caused by Nematodes:
Foliar nematodes, Aphelenchoides spp.
These plant-parasitic worms attack virtually all plant parts and may cause leaf lesions, yellowing, necrosis and leaf drop, and bud malformation. Water-soaked blotches appear on the underside of the leaf but soon involve the entire leaf. The whole plant may become stunted. The nematodes live and move in water films.
Reducing leaf moisture and removal of infected tissues, debris, or plants is important.
Diseases caused by Physiological/Environmental Factors:
Corky scab or oedema, physiological.
Raised, scab-like swellings appear on the underside of leaves. These first appear as water-soaked blisters and may turn rusty-brown with age. This condition is often associated with inadequate light levels as well as overwatering, especially during periods of cloudy, cool weather.
This problem can be minimized by careful attention to soil moisture levels, especially during periods of cloudy, humid weather.
Aphids, Aphis gossypii.
The melon aphid and probably other species occasionally infest begonia. They may be controlled by spraying with insecticidal soap, ultra-fine horticultural oil or malathion, which are among the compounds registered for use against this pest in Connecticut, when needed. Imidacloprid applied as a systemic to be taken up by the roots will also provide season-long control. Consult the label for dosage rates and safety precautions.
Black vine weevil, Otiorhynchus sulcatus.
Another common pest is black vine weevil. The larvae of this weevil often injure tuberous begonias in nurseries and ornamental plantings. The grubs devour the small roots and feed inside the tubers, weakening them. Severely injured plants die. The 1/2" long adult weevil is black, with a beaded appearance to the thorax and scattered spots of yellow hairs on the wing covers. Only females are known, and the adults are flightless. They feed nocturnally, notching the margins of the foliage. The legless grub is white with a brown head and is curved like grubs of other weevils. Adults and large larvae overwinter, emerging from May - July. The adults have to feed for 3-4 weeks before being able to lay eggs. Treating the soil with insect pathogenic nematodes may control the larvae, and should be the first line of defense. Acephate and fluvalinate are among the compounds registered for control of this pest in Connecticut, and should be applied when adults are feeding and before they start laying eggs. The usual timing for these foliar sprays is during May, June and July at three week intervals. Insecticide resistance is very common; be aware that adults may appear to be dead following contact with fluvalinate, but may recover from poisoning within a few days. Consult the label for dosage rates and safety precautions.
See African violet for indoor plants. On begonias used as annuals outdoors, insecticidal soap, ultra-fine horticultural oil as well as malathion, which are among the compounds registered for use against this pest in Connecticut, can be used to control this insect if needed. Imidacloprid applied as a systemic, to be taken up by the roots, will also provide season-long control. Consult the label for dosage rates and safety precautions.