The Causes of Yellow Leaves and Brown Spots on Hydrangeas

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The Causes of Yellow Leaves and Brown Spots on Hydrangeas
Hydrangea colours vary according to the pH of the soil. (hortensia pale image by daniel sainthorant from Fotolia.com)

Five different diseases that cause yellow leaves and brown spots commonly plague hydrangeas, according to Auburn University plant pathologists Austin Hagan and Jackie Mullen. Cases of root rot can also cause yellow leaves and brown spots in hydrangeas. Root rot occurs when hydrangeas are planted in poorly draining soils or are overwatered, according to the US National Arboretum. Yellow leaves may be a symptom of iron deficiency. The higher the pH of the soil (the more alkaline), the less available iron is to the hydrangea. You can add iron to the soil, or increase its acidity (lowering the pH) by adding aluminium sulphate or organic mulch.

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Cercospora Leaf Spot

The Cercospora hydrangeae fungus can attack your hydrangeas. This fungus appears as circular brown or purple spots on the hydrangea leaves. The spots may turn tan or light grey in the centre and are surrounded by a brown or purple ring. Removing diseased leaves, adding nitrogen and surface watering can slow the spread of Cercospora leafspot. Outdoors, rainfall and splashing water spread the fungus.

The Causes of Yellow Leaves and Brown Spots on Hydrangeas
Always remove leaf debris and prune diseased leaves. (hydrangeas image by Ruslana Stovner from Fotolia.com)

Anthracnose

The Colletotrichum gloeosporioides fungus attacks hydrangea leaves and blooms. The fungus prefers heavily fertilised hydrangeas and hot, wet weather. Brown spots on the hydrangea leaves may appear like bullseyes. As these brown spots grow, they become angular blotches. This fungus comes from the spores of wet plant debris. Removing fallen and diseased leaves can help slow the spread of anthracnose, and protective fungicide treatments every two weeks may help.

The Causes of Yellow Leaves and Brown Spots on Hydrangeas
Some diseases attack the flower petals as well as the leaves of hydrangeas. (verblühende Hortensie image by ernstboese from Fotolia.com)

Botrytis Blight

Botrytis blight is a grey mould that sprouts after cloudy, humid and rainy weather. It usually affects flower petals but this reddish-brown spotting can affect hydrangea leaves. Clearing away debris often and treating with fungicides can help prevent the blight.

The Causes of Yellow Leaves and Brown Spots on Hydrangeas
Wet leaf debris can carry many fungal spores. (Leaves image by Simon from Fotolia.com)

Powdery Mildew

Powdery mildew spreads rapidly in greenhouse hydrangeas but isn't as obvious in landscape hydrangeas. The cottony, white fungus first appears as small white spots on the leaves. The E. polygoni fungus that causes powdery mildew may also lead to yellow spots and purple blotches on hydrangea leaves.

The Causes of Yellow Leaves and Brown Spots on Hydrangeas
Rainfall can reduce powdery mildew. (rain image by Aqua Pixie from Fotolia.com)

Phytophthora Root Rot

Excess water can cause phytophthora root rot in hydrangeas. Yellow leaves and wilting are signs of this root fungus. Plant hydrangeas on raised beds to prevent root rot. Fungicides can help prevent the disease but cannot cure it once it's established.

The Causes of Yellow Leaves and Brown Spots on Hydrangeas
Plant your hydrangeas on raised beds or in well-draining soil. (Tellerhortensie blau image by EDEN from Fotolia.com)

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