Cercospora leaf spot
Cercospora leaf spot is a common disease in beetroot and silver beet but is usually unimportant in well-managed crops. It may be a significant problem in crops grown for baby-leaf production, because the foliage is the saleable product.
Cause
The fungi Cercospora beticola, C. capsici, C. nasturtii, C. canescens and C. coffeicola.
Cercospora spot in avocado is caused by the fungus Pseudocercospora purpurea.
Symptoms
Small, brown flecks develop with a reddish border, expanding to circular spots about 4mm wide with an ashy-grey centre. This tissue becomes thin and brittle, and often drops out, leaving a ragged hole.
In capsicums small, round water-soaked lesions develop on leaves, petioles and stems. The lesions enlarge and have light brown centres with dark brown-red margins. As lesions expand, an outer water-soaked area and dark ring may form beyond the original lesion margin, so that the lesion centre becomes surrounded by concentric rings. With age, the lesion centres dry out and crack.
In carrots cercospora leaf spot is more severe on the young leaves developing initially as small necrotic flecks that enlarge to form circular, tan or grey spots. Spots may coalesce during humid weather to blight the entire leaf. Sunken, elongated spots may also occur on leafstalks.
A sooty to dark olivaceous mould develops on the lower leaf surface of okra plants. As the disease progresses, leaves roll, wilt and fall from the plant.
In avocado and coffee crops both leaves and fruit develop dark brown lesions.
How it spreads
The fungi survive on undecomposed beet residues in the soil, on weed hosts and on beet seed. Hosts include beetroot, silver beet, sugarbeet, spinach and several Atriplex and Chenopodium weed species. Leaf spot is favoured by warm, wet weather. Severe outbreaks generally require a period of showery weather.
Infection from germinating fungal spores occurs via penetration of leaf stomata by fungal hyphae. Spores spread in wind, rain, irrigation or via mechanical means. The fungus is likely to carry over to new crops on infected crop debris.
Crops affected
Bean, beetroot, capsicum, okra, silver beet, watercress, carrot, avocado and coffee.
Control
Apply the recommended fungicides, particularly during warm, wet weather. Rotate beet crops with other non-host vegetables. Control weeds, particularly Chenopodium weeds like fat-hen, in and around beet crops.
Specific controls are not usually required for capsicum.
Plant only high-quality seed. Do not plant seed potentially infected with Cercospora.
Destroy infected crops promptly after the final harvest and before replanting to minimise disease spread to subsequent crops.
Chemical registrations and permits
Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.