Dahlia Diseases Table of Contents

This table of contents provides a preliminary list of diseases with abstract that may afflict dahlias.

1. Bacterial Wilt

Causes stem drooping and wilting. Infected stems have a wet soft rot near the soil. Destroy infected plants and rotate plantings.


2. Bacteriosis

Causes browning and softening of the stem. The pith is moist and blackened with the rot extending into the bark. A foul odor is associated with the disease. The roots may be infected.


3. Botrytis Blight

A popular name is gray mold. A common fungus. Spores germinate and penetrate tender, weakened, injured, aging and dead tissues during periods of high humidity in temperatures ranging from 32 to 85 degrees fahrenheit. They initially cause brown spots which continue to enlarge in humid atmosphere and eventually turn to a tan to gray fuzzy mold. Remove affected parts and treat with fungicide.


4. Canker and dieback diseases

Cankers are dead portions of plant stem tissue. Caused by bacteria or fungi that enter the stem through wounds, stomates, or nectarthodes in flowers. The most common bacterial canker is fire blight caused by Erwinia amylovora.


5. Crown Gall

Caused by a soil bacterium -- Agrobacterium tumefaciens. Causes large abnormal growths at the base and or on tubers of the infected plant. The plants are stunted and the shoots spindly. Infected plants, including tubers, should be deystroyed.


6. Powdery Mildew

Causes white or grayish powdery mold. Worse late in the season. Badly infected leaves distort, droop, and drop. While unsightly, powdery mildew is seldom fatal.


7. Stem Rot

Dahlias growing in heavy wet soil, and infected with stem rot, typically wilt and die. A white mold encircles the stem at the soil line. Treat by improving drainage and not crowding plants. Rotate plantings.


8. Verticulum Wilt

Causes brown or black streaking in the conductive tissue of the stem. Plants wilt and die. Stored tubers rot if infected. Destroy infected plants and rotate plantings.


9. Viruses

Causes a variety of leaf mottling patterns, necrotic and ring spots on and bubbling or distorted leaves, stem streaking, stunting, and wilting. Diagnosis often is misleading, if based on symptoms alone. Infected plants can also be carriers showing no symptoms whatsoever. Viruses cannot be controlled with pesticides. Thrips is the most common vector. Controlling thrips will help prevent spread of viruses.



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