Leaf gall caused by fungus on a cranberry plant

Like all food crops, cranberries are susceptible to various fungal diseases. Your best weapons against these are best planting practices, which help prevent disease in the first place.

This is especially important, as there are no fungicides approved for home use for many plant diseases.

These best practices are aimed at producing strong, healthy plants that can withstand disease—and at avoiding situations that contribute to the development of disease. They involve keeping plants clean, dry, and undamaged.

Mulch: Mulch can help with water retention. The ideal mulch for cranberries is sand. This has the added benefit of not retaining moisture, so it’s inhospitable to fungal infections.

Watering: Water your cranberries deeply about once a week (check the soil for dryness). Do not over-water. Soggy soil invites disease.

Other best practices include:

• Buy healthy, disease-free plants from reputable sources.
• Plant your cranberries in full sun.
• Plant in sites with good drainage.
• Check plants regularly for signs of disease.

Common cranberry diseases

Here are some of the usual culprits that might infest your cranberry plants. Remember, it’s important to remove infected plant material (leaves, roots) to prevent the spread of disease once it’s found its way onto your plant.

Botryosphaeria Fruit Rot and Berry Speckle

Cause: Fungus

Symptoms:

• small, light-colored lesions on the berry skin
• on green berries, lesions appear as red rings
• berries develop a watery rot after harvest

How it spreads:

• rain or wind

Treatment:

• remove and discard infected plant material

Prevention:

• buy disease-free plants
• avoid watering from above; water at soil level
• make sure plants have good air circulation and lots of sun

End Rot

Cause: Fungus

Symptoms:

• soft watery rot with clear boundaries; can start at either end of the berry
• rot eventually covers entire berry, making it soft
• upper leaf surfaces may develop red-brown spots with tan or gray centers and black borders; areas beyond the black borders turn bright red
• leaf drop

How it spreads:

• fungus overwinters in stems, vines, dead leaves, and rotting fruit

Treatment:

• remove and discard infected plant material
• do not compost infected plant material

Prevention:

• usually shows up after berry harvest; refrigerating berries can stop development of the infection

False Blossom

Cause: Phytoplasma (single-cell organism) spread by leafhoppers

Symptoms:

• abnormal flowers that do not set fruit
• blossoms may be dark pink or streaked with red
• flower stalks stick straight up instead of arching
• infected uprights (stems) may have a witches’ broom appearance

How it spreads:

• blunt-nosed leafhopper spreads infection from one plant to another when it feeds on the plant

Treatment:

• dig out infected vines, including the roots

Prevention:

• set traps for leafhoppers
• use floating row covers

Phytophthora Root and Runner Rots

Cause: Oomycetes (fungus)

Symptoms:

• plants not thriving
• weak, unproductive, or dead vines
• leaves turn yellow, then red, then collapse
• reduced flower and fruit production
• discolored roots

How it spreads:

• poorly drained soil
• overwatering

Treatment:

• remove and discard infected material
• apply fungicide as necessary
• fertilize plants to stimulate root growth

Prevention:

• plant in well-draining soil
• do not overwater
• increase drainage
• add sand to soil

Proventuria Early Leaf Spot

Cause: Fungus

Symptoms:

• red circles or rings on the upper surfaces of leaves
• marks darken as fungus multiplies

How it spreads:

• fungus overwinters in infected plant material
• wind, insects, and birds

Treatment:

• remove and destroy infected areas

Prevention:

• liquid copper fungicide
• plant disease-resistant varieties
• remove plant debris

Red Leaf Spot

Cause: Fungus

Symptoms:

• circular spots on the upper surface of leaves
• spots on young berries

How it spreads:

• fungus overwinters on infected leaves and stems
• high humidity, including fog and heavy rain

Treatment:

• remove and discard infected parts
• apply foliar fungicides

Prevention:

• spray with Bordeaux mixture after buds break in the spring
• avoid overfertilizing

Which diseases have you had to treat on your cranberries? Please tell us how you prevent and handle diseases. If you spot other symptoms on your cranberries that are not mentioned here, contact your local extension center or garden center for a consult—and please let us know what you discover by commenting below.