Phoma

Phoma is a large genus of fungi that behave as opportunistic pathogens on a wide range of vegetables, ornamentals and field crops grown in Australian gardens. Depending on the host and species involved, Phoma causes leaf spots, stem cankers, blights, or rots of tubers and fruit. It is a common cause of ray blight in chrysanthemums, gangrene and dry rot in potato tubers, black stem and ascochyta-type blight on peas, and ripe rot on fruiting crops.

On leaves, Phoma produces roughly circular to irregular brown or grey-brown spots, often with a darker margin and sometimes a faint yellow halo. As spots age, tiny black pinhead-like fruiting bodies (pycnidia) can be seen scattered across the dead tissue, especially under magnification or with the surface kept moist. On stems and crowns, infection shows as sunken, dark brown to black cankers that can girdle the stem, causing wilting and collapse of growth above the lesion. On stored tubers or fleshy roots, Phoma causes a firm, dark, sunken dry rot that slowly enlarges in storage.

The disease is favoured by cool, wet or humid weather, and spreads by wind-blown or splashed spores released from infected plant debris, seed, or infected tubers used for planting. It is most active in autumn, winter and spring in temperate parts of Australia, though it can appear whenever conditions stay damp for extended periods. Infection is worse where plants are stressed, overcrowded, or where dense foliage keeps humidity high around the base of plants.

Management relies on reducing humidity and removing sources of infection. Space plants to allow airflow, avoid overhead watering late in the day, and remove and destroy infected leaves, stems and fallen debris rather than composting them. Practise crop rotation for susceptible vegetables, use certified disease-free seed or seed tubers, and avoid handling or harvesting plants when foliage is wet. Cutting out and destroying cankered stems as soon as they're seen helps limit spread within a planting.