Gardenias - Knowledgebase Question

laguna Niguel, Ca
Question by medebella
May 24, 2010
My gardenias are going yellow and black. I don't think the problem is too much water . They are in filtered sunlight.
Can they be saved?

Answer from NGA
May 24, 2010


Several different diseases can cause black leaves on a gardenias, but sooty mold can also show up as a thin black coating on the upper surfaces of the leaves. Sooty mold is caused by insect feeding. As the pests feed they exude a sticky substance called honeydew. When this ages and weathers it turns black. If you can scrape the black substance off the tops of the leaves, it's sooty mold. You'll need to control the insect pests to stop the sooty mold from forming.

Diseases that can cause blackened leaves include Bacterial Leaf Spot. Small, round ovoid spots on young, tender leaves are the first sign of bacterial leaf spot. As the spots enlarge, the center is at first pale yellow and later becomes reddish-brown surrounded by a yellow halo. Margins of the lesions are thickened and water-soaked in appearance. Spots may coalesce to form large, irregularly shaped spots. Severe infection may cause defoliation. To avoid the disease, avoid overhead watering.

Rhizoctonia Leaf Spot - Leaves infected with this fungal leaf spot disease have tan to brown spots up to 1/4 inch in diameter. Spots are circular and zoned. The disease begins on the older leaves and spreads upward when the plants are watered excessively or when air circulates poorly because of overcrowding. Diseased leaves should be destroyed.

Phyllosticta Leaf Spot - These fungi cause spots of various sizes on leaves throughout the year. Spots may be small, dark-brown necrotic areas surrounded by a yellow halo. In severe cases, premature leaf drop may occur.

A foliar fungicide sprayed at regular intervals will help control these diseases. regular intervals.

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