Botrytis Grey Mould on Cantaloupe — Identification and Control

Botrytis cinerea, commonly known as grey mould, is an opportunistic fungal pathogen that attacks cantaloupe at multiple points — flowers, stems, and developing or ripe fruits. It is especially problematic in polytunnels and greenhouses, or in cool, damp outdoor seasons, where high humidity provides ideal conditions for the spores to germinate and spread.

Recognising Botrytis on Cantaloupe

Botrytis infection typically begins on dead or dying plant tissue — spent flowers, leaf stalks, or damaged stems — before spreading to adjacent healthy tissue. The characteristic sign is a grey, fluffy, powdery mould growth on the infected area. Unlike powdery mildew, which is dry and chalk-white, botrytis mould is distinctly grey and appears fluffy or dusty in texture. Affected tissue beneath the mould is wet, soft, and brown. If disturbed, a cloud of fine grey spores will puff up — these spread rapidly on air currents to nearby plants.

Why It Attacks Certain Plants First

Botrytis is a wound pathogen — it almost always enters through damaged, dead, or senescent tissue rather than healthy green stems. Plants that are stressed, over-fertilised with nitrogen (producing soft, weak growth), recently pruned, or growing in stagnant humid air are the most vulnerable. In a polytunnel with poor ventilation, botrytis can progress through a planting remarkably quickly once it is established.

Management: Remove and Improve Airflow

The first step is immediate removal of infected tissue. Use clean, sterilised scissors to cut away affected stems, flowers, and leaves, cutting several centimetres back into healthy tissue. Place removed material in a bag and dispose of it in the bin — not the compost. Do not work in the crop when leaves are wet and avoid splashing water onto the foliage. Open vents and doors in polytunnels to improve airflow and reduce humidity, particularly at night when temperatures drop and condensation forms.

Preventing Botrytis Through Cultural Practice

Remove dead flowers promptly — these are the most common entry points on cantaloupe. When training and pinching vines, allow cuts to heal in dry conditions and avoid creating large pruning wounds in cool, damp weather. Keep the growing area free of dead leaves and plant debris. Space plants to allow generous airflow between them. In a greenhouse, reduce humidity by heating to above dew point in the evenings during cool weather, or use a small fan to keep air circulating.

Organic Spray Options

Potassium bicarbonate sprays can suppress botrytis when applied as a preventative measure in high-risk conditions. Copper-based products have some activity but are less targeted for botrytis than for downy mildew. Trichoderma-based biological fungicides are an effective and increasingly available organic option — Trichoderma species are beneficial fungi that colonise plant tissue and compete aggressively with Botrytis. Apply to pruning wounds and at the base of the plant at planting time in high-risk conditions.