Treating Garden Fungi

Photo: Black spot
Use nontoxic fungicides to treat wet-weather gardening pests like rust, black spot and powdery mildew

This spring has not been typical by any means. Outside my window it has been cool and wet without a whole lot of sun. Unfortunately, these conditions can lead to a few fungal problems in the garden. While fungal diseases can reduce the growth of plants, their main side effect tends to be diminishing the appearance of the garden. Here’s how to treat three common pests with earth-friendly fungicides.

Black spot

A fungal disease that thrives in damp humid weather and can show up on the leaves of roses and other plants. It appears as black spots with perforated edges, which can reach 14mm in diameter. Remove infected leaves and spray the entire plant with a nontoxic fungicide. Prune roses to increase airflow through the plant.

Powdery mildew

This group of fungi can affect a wide range of plants and thrives in damp humid weather. It first appears as single white or grey fuzzy spot on leaves or fruit and can quickly spread by releasing fungal spores. Controlling this problem is actually fairly simple. Space plants so there is good air circulation and remove any debris from the soil surface. Dust the leaves with a biodegradable garden sulphur such as Green Earth Garden Sulphur to control the spread of fungal spores. Sulphur makes the surface of the leaves more acidic. This change in pH helps to control the existing fungi and reduce the spread.


Look out for reddish brown pustules or hair-like structures on the underside of plant leaves and on stems. This fungal disease will not usually kill the plant but is very hard to treat and is pervasive. To manage this problem, practice good garden hygiene (remove debris from the soil surface on a regular basis) and prune plants to allow good air circulation. When rust appears, remove all infected leaves and dust the plant with a nontoxic garden sulphur or copper spray to stop any remaining fungus from germinating. Repeat dusting once a week and after any significant rain.


Green gardener Mark Cullen is a radio and TV personality, author of 18 gardening books and answers thousands of questions at