|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/|
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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