| Vinca stem canker, Phoma exigua, is a fungal
disease that is almost always prevalent during very
rainy seasons. This fungal disease of periwinkle or
ground myrtle causes shoot tips to become dark
brown changing to nearly black as the disease
progresses. Stem canker (blight) may be so severe
that attempts to maintain vinca plantings fail, and
the plants have to be removed and replaced with
other non-susceptible ground covers. Damage is
most prevalent in cool wet weather in spring and in
autumn when soil moisture is high. Newly planted
beds, where plantings are thin and not well
established, are very susceptible.
Incidence of the disease tapers off during the hot dry
weather of summer; however, in prolonged periods
of cool wet weather, new infections can occur. Stem
canker can also be a severe problem in nurseries
where plants are grown under mist propagation.
The fungus grows and persists indefinitely in moist
soil and plant debris. It is not unusual for new plants
to appear healthy when purchased only to become
diseased when planted. The causal fungus survives
in infested plant debris which is difficult to remove.
Symptoms and Diagnosis
Soon after new green shoots appear in spring, vinca
stem canker, Phoma exigua, causes shoot tips to
become dark brown, wilt, and die back to the soil
surface. Some affected stems are nearly black. After
several weeks, entire clumps of plants may wither
and die. Small, black, dot-like, fruiting bodies
develop on the surface of the diseased stem tissue.
Life Cycle
The sexual stage of this fungus is known as
Diaporthe vincae. The spores are produced during
very rainy seasons. The fungus grows saprophytically
throughout moist soil and commonly colonizes dead
and dying plant material. The fungus is believed to
overwinter as dormant mycelium and as specksized,
brown to black fruiting bodies (pycnidia) on
the dead stems and leaves.
The cycle of infection, disease development,
symptom expression, and re-infection may be
repeated as long as cool, damp conditions prevail
and the soil moisture remains high.
Integrated Pest Management Strategies
1. Removal of debris. Remove infected plants and
prune out diseased parts of the plant in the fall
and spring. Discard this material but do not
compost. Thin planting in the fall during dry
weather to remove dense growth.
2. Purchase disease-free plants. For new beds, select
and plant only vigorous, disease-free stock
purchased from a reputable nursery.
3. Water. After transplanting, apply only enough
water to maintain vigor. Avoid frequent watering
if the soil is sufficiently moist for good growth.
4. Fungicides. Under conditions in the spring of
extensive and frequent rainfall, fungicides can be
applied to diseases beds every 7–10 days.
Application should cover all leaves and stems to
runoff. Through coverage is essential to be effective. | 
