Leaf spot

Leaf spot diseases can be caused by many different kinds of fungi and can occur on most plants - food producing and ornamental. Some leaf spot diseases are harmful to plants in that they can greatly reduce the plant's vigour and ability to survive stresses.

However, most are not harmful and simply cause small brown spots on the leaves that have minimal impact on the plants ability to photosynthesize. Positive identification of the specific fungus causing the leaf spots is often not a simple task, as many types of fungi can cause leaf spots.

Symptoms:

• Usually small brown/ yellow/tan/black spots on the leaves depending on the time of year/ fungi/ host plant; spots are randomly dispersed.
• Spots are most often circular, but can be angular (straight edges).
• Most spots remain small, however, they may enlarge and eventually kill the leaf causing it to fail prematurely; repeated years of leaf loss can weaken plants.
• Sometimes the dried tissue in the center of the spots will fall out leaving a hole in the leaf - often called "shot hole" disease.
• Some leaf spots can have concentric rings around the spots - some don't have rings/ margins around the spot may be a different colour than the spot.

Life Cycle:

• Extremely variable depending on the fungus.
• Some fungi have a simple life cycle: over-wintering, then producing spores in spring/summer which infect leaves, then over-wintering again.
• Some fungi have a complicated life cycle involving more than one type of plant (alternate host).
• Most infections occur in late spring when the new growth is occurring on trees and shrubs but the infection may not be visible for weeks until the spots enlarge enough to be seen.
• Fungal infections usually occur after a long rainy period with cooler temperatures.
• Some fungi will produce a second type of spore which can result in further infection and rapid spread of the disease.

Control:

• Most fungal spores require humid/ damp conditions for a prolonged time period (1 - 3 days) in order for the spores to germinate; anything that will reduce these conditions will help to control the fungus:
  • Improve air circulation if plants are dense or overcrowded (thinning, pruning)
  • Water in the morning so the foliage is dry going into the evening
  • Water the soil so as to not wet the leaves
  • Space plants further apart so they are not crowded
  • Avoid working with the plant when it is wet as this can spread the fungal spores
  • Disinfect tools such as pruners in between cuts to avoid spread
• Most fungi will have a resting stage when spores over-winter on plant debris at the base of the plants. Remove and destroy, or hot compost the leaves to help break the cycle.