DiseaseMedium Severity

Diplocarpon Theumenii (Hawthorn Leaf Spot)

Diplocarpon mespili

A fungal leaf spot disease affecting hawthorn, photinia, and related plants. The disease is favored by cool, wet weather and can cause severe defoliation, reducing landscape screening value and increasing plant susceptibility to cold injury and other stresses.

Peak Season: Overwintering: In twig lesions and fallen leavesPrimary infection: Spring when spores splash to new leavesOptimal conditions: Cool, wet weather (60-80°F with 12-24 hour leaf wetness)Secondary cycles: Repeated throughout growing seasonYear-round activity: Possible in mild winter climates on evergreen hosts
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Diplocarpon theumenii causing hawthorn leaf spot with characteristic red spots and maroon blotches

Identification & Symptoms

What to Look For

Small, circular, often bright red spots on leaves
Spots may grow together forming large maroon blotches
Heavy defoliation in severe infections
Early leaf drop reducing screening value
Isolated plants often remain healthy due to limited spore dispersal
Lesions may persist on young, green twigs over winter

Potential Damage

Severe defoliation reducing photosynthetic capacity
Loss of landscape screening and aesthetic value
Increased sensitivity to cold injury
Enhanced susceptibility to other diseases and insects
Reduced plant vigor and health
Economic losses in ornamental plantings

Lifecycle

Fungus persists through winter in lesions on young twigs or in acervuli on fallen leaves. Primary infection begins when spores splash onto new leaves in spring. Multiple secondary infection cycles create new generations of conidia throughout the growing season, causing progressive disease buildup.

Professional Treatment Approach

1

Fungisol applications during cool, wet weather periods

2

Preventive treatments before symptom development

3

Remove and destroy fallen infected leaves

4

Improve air circulation around plants

5

Avoid overhead watering to reduce leaf wetness duration

Recommended Mauget Products

Fungisol

Professional-grade treatment specifically effective against diplocarpon theumenii (hawthorn leaf spot).

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