Image credit - http://www3.syngenta.com/

If you have grown sunflowers before then the chances are that you have experienced at least one of them suddenly wilting to a quick and ultimately death. Yes they have been adequately watered, but the tell-tale signs of the lower leaves collapsing and early flower bud formation are undeniable evidence that something is desperately wrong.

If it isn't something mundane like a snapped stem or catastrophic insect damage then your sunflower has most likely been struck down by the dreaded Phoma Black Stem disease.

Phoma Black Stem disease

Image credit - http://www.plantmanagementnetwork.org/
Phoma Black Stem disease - Phoma macdonaldii, is the most common of all sunflower diseases and is the result of a soil borne fungus. The infection is characterized by large dark lesions on the stem, low down near the soil level - but not always. The infection usually starts on the leaf and follows the petiole to the stalk. While the sunflower is young the infection will often go unnoticed especially if there are other plants growing nearby which hide the damage from view. The large patches on the stalk become most noticeable after petal drop, but by then the damage is done and the spores have been spread.

The fungus would have originally been brought into the garden on infected sunflower seed and will overwinter in plant debris. The following spring it is spread to your new season sunflowers by splashing rain or by biting insects such as leaf miners and stem weevils. Incidentally the larvae of stem weevils can spread the fungus further while tunneling in the stalk.

It is of course quite possible for infected plants to recover but they may produce smaller flower heads and less seed as a consequence. In the case of giant sunflower cultivars, they will not go as tall as expected. This is because the stalk lesions are on the surface only and the inner pith is not destroyed.

Control of Phoma Black Stem disease

Sadly there is no fungicide treatment available for Phoma Black Stem disease, but this doesn't mean you should not spray as your plants weakened state may leave them as risk from secondary fungal infections. It is recommended to plant sunflowers in soil where sunflowers have not been grown for at least four years. Also consider spraying with a systemic insecticide every fourteen days to control damage from biting insects.

However the best method of control Phoma Black Stem disease is to plant Phoma Black Stem tolerant hybrid sunflower seeds. Unfortunately at the time of writing this article the viability of Phoma Black Stem tolerant hybrid sunflower seeds are still being researched, and are not yet available to the general public.

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