---
title: Phoma
canonical: https://fernland.com.au/problem-solver/phoma/
dateModified: 2026-08-07 09:39:19
summary: "Phoma is a fungal pathogen genus causing leaf spots, stem cankers, blights and tuber or fruit rots on a wide range of ornamental and horticultural crops."
type: article
keywords: [Phoma, Phoma spp.]
---

# Phoma

> Phoma is a fungal pathogen genus causing leaf spots, stem cankers, blights and tuber or fruit rots on a wide range of ornamental and horticultural crops.

*Phoma spp.*

Phoma is a large genus of fungi that behave as opportunistic pathogens, attacking stressed, wounded or senescing plant tissue on a broad range of hosts including chrysanthemum, dahlia, pea, potato and many other ornamentals and vegetables. Different named diseases within the genus (such as ray blight, gangrene of potato tubers and ascochyta-type blight of pea) share a similar biology even though the symptoms and hosts differ.

On infected tissue the fungus produces small, dark brown to black lesions on leaves, stems or fruit that often enlarge and merge, sometimes showing a target-like ring pattern or a pale halo. Within these lesions, tiny black pinhead-sized fruiting bodies called pycnidia develop, embedded in the dead tissue; these are visible with a hand lens as raised black specks and exude masses of spores in wet conditions. Stem infections can girdle the base of the plant, causing collapse or dieback above the lesion, while on tubers or fleshy roots the fungus causes a firm, dry, sunken rot that can extend inward as a brown or black discoloured cavity.

Disease development is favoured by cool, wet or humid weather, overhead irrigation, and plant stress from wounding, poor nutrition, waterlogging or frost damage. Spores spread by rain splash, irrigation water, handling, and contaminated tools or planting material, and the fungus can survive in plant debris, soil and infected propagation stock between seasons.

In Australian production nurseries and cut flower or vegetable operations, Phoma problems are most common during prolonged wet periods in cooler months or under dense, poorly ventilated crops. Management relies on sanitation — removing and destroying infected plant material and debris, avoiding overhead watering late in the day, improving airflow and spacing, using clean or certified disease-free planting stock, and rotating away from susceptible crops where soil-borne inoculum has built up. Minimising plant stress and physical wounding at harvest or transplanting reduces infection sites for the fungus.

## How to identify

- **Leaf and stem lesions:** brown to black spots, sometimes with concentric rings or a yellow halo, which can enlarge and coalesce, leading to leaf yellowing, wilting and dieback.
- **Stem cankers:** sunken, dark lesions at or near the plant base that can girdle stems, causing sudden collapse of the top growth while roots remain firm.
- **Tuber, root or fruit rot:** firm, dry, sunken lesions that develop into a brown-black internal rot, distinct from the soft, wet decay typical of bacterial soft rots.
- **Confirming the pathogen:** use a hand lens to check lesions for tiny black pinhead fruiting bodies (pycnidia) embedded in the dead tissue; in humid conditions these may ooze spore masses, which is a strong indicator of Phoma rather than a purely physiological or insect-related mark.
- **Lookalikes:** Ascochyta and Septoria leaf spots produce similar dark lesions with pycnidia and are difficult to distinguish without laboratory diagnosis; bacterial rots lack the black fruiting bodies and instead produce a soft, slimy, foul-smelling decay, which helps separate the two in the field.

## Registered products we stock

- [Mancozeb DG Fungicide](https://fernland.com.au/barmac-mancozeb-750/) — Amgrow Specialty Agriculture
- [Penncozeb 750DF Fungicide](https://fernland.com.au/mancozeb-750df-fungicide/) — UPL
- [Switch Fungicide](https://fernland.com.au/switch-fungicide/) — Syngenta
- [Cheers Weathershield Contact Fungicide](https://fernland.com.au/cheers-weathershield-contact-fungicide/) — Colin Campbell Chemicals

## Image credits

- [Photo: Dr. Jan Hinrichs-Berger, Landwirtschaftliches Technologiezentrum Augustenberg, Karlsruhe. via Wikimedia Commons (CC BY-SA 4.0)](https://commons.wikimedia.org/wiki/File:Pastinaca_sativa_-_Phoma-F%C3%A4ule_-_Calophoma_complanata-2_-_Hinrichs-Berger,_Augustenberg.jpg)
- [Photo: Dr. Jan Hinrichs-Berger, Landwirtschaftliches Technologiezentrum Augustenberg, Karlsruhe. via Wikimedia Commons (CC BY-SA 4.0)](https://commons.wikimedia.org/wiki/File:Pastinaca_sativa_-_Phoma-F%C3%A4ule_-_Calophoma_complanata-1_-_Hinrichs-Berger,_Augustenberg.jpg)
- [Photo: Svarði2 via Wikimedia Commons (CC BY-SA 4.0)](https://commons.wikimedia.org/wiki/File:Phoma_in_%22Rau%C3%B0ar_%C3%ADslenskar%22_potato.jpg)
- [Photo: Howard F. Schwartz, Colorado State University, Bugwood.org via Wikimedia Commons (CC BY 3.0 us)](https://commons.wikimedia.org/wiki/File:Phoma_macdonaldii_on_sunflower_plant_-_1.jpg)