Protecting Pawpaw Plants Against Black Spot Disease

black spot disease (Asperisporium caricae) on pawpaw

black spot disease (Asperisporium caricae) on pawpaw

If you live in south eastern Queensland and grow pawpaw, it’s worth a quick health check right now. Mild, calm, showery, humid winter weather is perfect for observing outbreaks of pawpaw black spot disease. But how best to manage this common problem?

The town of Wamuran in SE Qld is ground zero for the introduction and spread of this exotic fungal disease affecting pawpaw plants and their fruit. If weather permits, this systemic disease can cause the total loss of fruit and foliage. Plants look awful and crops may be lost. While plants rarely die, they don’t earn their keep in a food garden.

Wind, rain and bird’s feet spread the fungal spores from garden to garden. We can’t control the weather, but we can condition pawpaw plants to better withstand attack. Protecting pawpaw begins in autumn by spraying all parts of the plant – fruit, flowers, both sides of foliage and stems right down to the ground using a fungicide, starting in February.

Spraying

black spot disease (Asperisporium caricae) on pawpaw

black spot disease (Asperisporium caricae) on fruit

Gardeners will never eliminate pawpaw disease, whatever they do. Spraying is one step in a strategy which reduces damage to plants and helps to rescue your food. The department of agriculture in Qld recommends monthly spraying from February to October using copper-based fungicides but they recognise than no more than five applications in a year is safe.

Copper-based fungicides accumulate in soil, and copper can affect beneficial mycorrhizal fungi. Copper is also translocated into fruit. A little copper is vital to human health and we consume it in everyday foods. Up to 12mg of copper eaten each day is the limit suggested by the World Health Organisation. An adult weighing 60 kilogrammes contains approximately 0.1g of copper – that’s not much to sustain. I advise against the routine application of copper fungicides over extended periods.

Copper hydroxide is the active ingredient and it will be named on the back of the pack. It is sold under various brand names, including Kocide and Fungus Fighter, and it may come in either powdered or liquid form. Copper oxychloride has a similar effect, however that formula, which persists longer in soil and is familiar to older gardeners, is not permitted in organic gardens.

Spraying with wettable sulphur (not dusting sulphur or flowers of sulphur, which are insoluble formulas) also works. It is an acceptable organic input, it doesn’t impact on soil health, plus it can be applied monthly from February to October. It is simple, effective and safe. There is one limitation – do not apply wettable sulphur when the temperature is forecast to reach or exceed 30C. Sulphur spray, like all sprays, is best applied in early morning on a calm, dry, cloudy day. 

black spot disease (Asperisporium caricae) on foliage

black spot disease (Asperisporium caricae) on foliage

The value of horticultural hygiene

Day to day control involves regularly removing severely infected leaves and fruit and all fallen foliage all year round. These are concentrated sources of disease spores and reinfect plants. Pick over plants before spraying and you’ll use less fungicide to cover them.

If plants are mulched, this will accumulate fungal spores. Remove old mulch at the end of autumn and replace it with fresh. Old mulch may be composted, just remember not to spread this under your pawpaw.

Infected fruit may rot rather than ripen on plants. Immerse infected fruit in water warmed to 45C and hold it at that temperature for 20 minutes. Heat treatment kills the spores.

The key to abundant, healthy fruit

'Southern Red' copes well with black spot disease

‘Southern Red’ copes well with black spot disease

A healthy pawpaw fights off disease more effectively, so love your plants. Give pawpaw one good weekly watering during dry weather, but avoid watering too frequently. In wet, poorly draining soil, roots may be attacked by decay fungi. I plant pawpaw in mounds 15cm above the ground level to assist drainage.

Modest but regular feeding builds their strength and resistence to disease. At the very least, ensure your plant receives a monthly supplement of potassium (sold in liquid, granular or crystalline form) and magnesium (sold as Epsom salts, the cheapest source being a supermarket). One teaspoonful of each is dissolved in 9 litres of water (they are chemically compatible and may be mixed together) and watered around the base of each plant. Apply monthly from February to October.

Well nourished pawpaws are more important for disease control than spraying. Potassium is the key nutrient for papaw production, followed by nitrogen, calcium, magnesium and phosphorus. Together, they not only produce delicious fruit, they strengthen plants. I provide these by sprinkling one handful of pelletised poultry manure per plant every month and supplement this with potash and Epsom salts during autumn and winter. It’s worth noting that pawpaw deficient in potassium, phosphorous and magnesium are susceptible to powdery mildew as well as black spot.

Spraying with sulphur or copper based compounds prior to the onset of the cool weather, feeding with poultry manure and watering that manure in with liquid seaweed really can help reduce the severity of black spot.

Jerry Coleby-Williams
Director, Seed Savers’ Network
Patron, National Toxics Network
11th July 2017