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I planted several rhododendrons in my garden this spring and now there are black spots on their leaves and a number of leaves are turning yellow. Do they have a plant disease?

Answer

Sounds like you have a fungal leaf spot disease, which usually shows up during hot, humid weather. Management of fungal leaf spot responds very well to healthy cultural practices, which include proper watering techniques, pruning and general cleanliness of area under the plant.

When watering, water the soil, not the above-ground plant parts. Make sure to water in the morning so the plant's leaves have a chance to dry off during the day. Allowing water to sit on leaves overnight, especially with the high humidity we have been experiencing, creates the perfect environment for black fungal leaf spot to thrive. Proper pruning techniques include creating air spaces within the center branches of the shrub to encourage air circulation. Cleanliness techniques include picking up the diseased leaves underneath and off of the diseased plant to deter spreading of disease.

The yellow leaves on the Rhododendron can mean a couple of things. The plants may be suffering from 'transplant shock'. Some plants are more sensitive to transplant shock and become stressed after being moved from a pot into the ground. A stressed plant will shed some of its leaves until the roots get established.

Secondly, if the plants are being watered too much, the leaves will turn yellow from lack of oxygen in the soil. Water deeply every 3-4 days to allow the soil to dry out between watering. Finally, you may be observing 'normal leaf shedding'. Broadleaf evergreens (Rhododendrons, Azaleas and Mt. Laurel) and needle leaf evergreens (Pine, Spruce and Fir) shed some of their leaves throughout the season, whereas deciduous shrubs and trees shed their leaves all at once in the fall.

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About Linda Lillie

Linda K. Lillie is the President of Sprigs & Twigs, Inc, the premier landscape design and maintenance, tree care, lawn care, stonework, and carpentry service provider in southeastern Connecticut since 1997. She is a graduate of Connecticut College in Botany, a Connecticut Master Gardener and a national award winning landscape designer for her landscape design and landscape installation work.

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