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Phoma black stem of sunflowers:
Potentially devastating in certain production fields
Phoma black stem is caused by the soil-borne fungal
pathogen Phoma macdonaldii, which occurs in some of the
major sunflower production areas of South Africa. Severe
infections on susceptible hybrids can cause up to 100% yield
losses in one particular field.
Disease survey results from the 2012/2013 and 2013/2014 sea-
sons conducted by the ARC-Grain Crops Institute revealed that
Phoma-like infected sunflower symptoms were observed in some
parts of Limpopo (Naboomspruit) and the North West Province
(Carletonville).
Epidemiology of the disease
The pathogen survives as pycnidia/perithecia on plant debris in
the soil and can also be seed-borne. The fungus is disseminated
through water (rainfall/irrigation) and by insects such as Apion
occidentale and Cylindrocopturus adspersus stem weevils, which
feed on sunflower. Infection can occur anytime during the growing
season when conditions are favourable (wet, humid and high
temperatures).
During the infection process, perithecia release ascospores, which
infect the leaves. Adult sunflower stem weevils feeding on the leaves
cause lesions whereas contaminated larvae spread the fungus
as they tunnel throughout the stem. Infected leaves die while the
pathogen spreads to the petiole and stem, forming irregular shaped
symptoms at the node. Infection can also occur through mycelia at
the basal part of the plant.
Symptoms of the disease
The disease is characterised by light to dark brown/black irregular
shaped lesions centred at the node (Photo 1).
The lesions originate from the leaf infections that progress down
the petiole to the stalk. Under favourable conditions, the leaf wilts,
petiole turns uniformly black and the stem lesions expand to form
a large, shiny, black patch with well-defined borders.
Small, circular fruiting bodies of the fungus are visible on the surface
of the stem. Pith damage may occur right on the petiole and as a
result, mid-stem lodging can occur (Photo 2).
Disease control
The fungus overwinters in infected sunflower debris on the soil
surface. Deep ploughing that burry plant residue as well as crop
rotation can accordingly reduce incidences and severity of the
disease. Crop rotation is important as it prevents disease build-up in
the field by interfering with the life-cycle of the disease on its host,
the sunflower.
Leaving crop residue on the soil surface will also encourage the
development of the disease. Control of insects such as sunflower
stem weevils can help minimise the spread of the disease. Dense
plant stands favour black stem development, therefore, optimum
plant densities and nutritional inputs can reduce infection. No hybrids
have been identified as being immune to the disease, although some
hybrids appear to be more tolerant than others.
For more information, contact Moses Ramusi at
MOSES RAMUSI and BRADLEY FLETT, ARC-Grain Crops Institute
1: Phoma infected plants showing black, irregular shaped lesions cen-
tred at the node.
2: Lodging observed on stalks severely infected with Phoma.
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ON FARM LEVEL Phoma black stem / Sunflowers / Fungal pathogen Integrated pest control