Impatiens With Yellow Leaves & Black Spots

Alternaria Leaf Fungus

• When coupled with excessive moisture, poor air flow due to overcrowding of impatiens can result in the growth of the Alternaria alternata fungus, which causes alternaria leaf spot blight. It can be differentiated from other leaf spotting ailments by the uniform size and roundness of its spots and the way it causes leaves to yellow and drop, according to Middletown, New York's "OnlineRecord" newspaper.

Controlling Alternaria

• Drip watering can help avoid watering foliage, which leads to fungal problems. But if overhead irrigation is necessary, watering early in the day reduces the occurrence of alternaria leaf spot, because it gives the plants a chance to dry out before the cooler temperatures of nightfall. It recommends removing spotted leaves as they appear, but only suggests using fungicides if warm, humid weather persists. Fungicides for impatiens recommended by the "Floriculture" newsletter of the University of Massachusetts are chlorothalonil, thiophanate methyl, copper compounds and mancozeb.

Necrotic Spot Virus

• One symptom of necrotic spot virus in impatiens is large, black-rimmed blotches, which are dead spots in the plant tissue. Other signs include stunted growth and chlorosis, a yellowing of foliage due to a lack of chlorophyll. The virus spreads from infected plants to healthy ones. A major carrier of the virus is the Western flower thrip (Frankliniella occidentalis), a winged insect that is less than 2-mm long. It spreads the disease to impatiens when it feeds on them, piercing plant parts and sucking out their cells. Larvae of Western flower thrips are clear when they hatch, then turn yellow and finally change to white before burrowing into soil for cocoon-like pupation taking 2 to 5 days. Then the brown or yellow adult thrips emerge.

Controlling Necrotic Spot

• There is no cure for a plant infected with necrotic spot virus, according to the University of Kentucky Cooperative Extension Service. Plants showing symptoms should be discarded immediately and should not be composted. Insecticidal soaps and a product called summer oil can be used to suffocate thrips, Clemson University says, but it is unlikely they will totally eliminate the insects.