Sunflowers bring a garden to life when they bloom, but a few diseases can bring them down. Jackie Buckley from the Morton County Extension Agency is here to tell us about them.
J.R.: Before we get into the diseases, what are some of the most common ways to control them?
Alyssa: One sunflower disease is nicknamed, "White Mold." What can you tell us about it?
J.R.: There's another one called "Phoma Black Stem." The name helps to explain it,but how does it develop?
Alyssa: And we have one called "Phomopsis Stem Canker." How can we identify it?
J.R.: Have we been seeing a lot of these diseases this year?
SCLEROTINIA STALK AND HEAD ROT - (WHITE MOLD) Sclerotinia sclerotiorum
Symptoms The entire plant wilts suddenly. Often occurs in patches in the field. A brown soft rot develops on the stalk, usually at the base but occasionally on the upper stalk. Later, the brown area develops a bleached appearance.
In wet weather a cottony white growth develops on the rotting stalk; later, tufts of cottony growth develop into hard black fungus bodies called sclerotia. Sclerotia also form inside the stalk. Roots may be stubby, brownish colored, and with cottony white growths on the surface. Later, sclerotia form on the surface and in the center of the taproot. Head infection, which comes from wind-blown spores, is less common than stalk infection. White cottony growths form on the head, the tissues are shredded and brown, and large black sclerotia form in the back (receptacle) of the head. Later, the heads develop a bleached appearance. Head rot and upper stalk rot occur in long periods of wet weather.
Control Plow down infected crop refuse in the spring. Use crop rotation; do not plant highly susceptible crops more than once in four years, including dry edible beans, mustard, canola (rapeseed) and sunflowers. Use at least a five year rotation for severely infested fields. Avoid high plant populations. Avoid planting next to a field known to have had white mold. Control broad-leaved weeds.
PHOMA BLACK STEM, Phoma macdonaldii
Symptoms Large black elliptical spots develop on the stem. These spots have a definite margin. When spots are numerous, the stalks become hollow and weakened and lodging may result. Soon after a spot begins to develop, the base of the petiole dies and the leaf hangs dead on the stem; Phoma spots at the axils of most leaves can result in a wilted appearance of the entire plant. Dark brown spots may also develop on the back of the head; a target pattern of black fungus fruiting bodies forms in these spots. A black area may develop at or below the soil line and girdle the plant. This is called Phoma girdling and is a common cause of early dying.
Survival and Spread Survives in crop refuse. A few seeds may be infected. Spread is by rain-splashed spores and stem weevils.
Control Crop rotation and stem weevil control may help.
Symptoms A large light brown stem lesion forms on the area surrounding the point of attachment of the leaf to the stem. Phomopsis resembles a "brown Phoma," but the lesion is much larger (up to 6 inches) and light brown instead of black. The stalk pith may be severely rotted. The disease frequently starts with infection of the lower leaves. Infection starts at the leaf margin, and a brown lesion spreads down one or more leaf veins towards the petiole and eventually to the stem. Foliar symptoms may be confused with Verticillium, but Phomopsis produces less chlorosis, and in Verticillium the leaf lesions are delimited by veins.
Survival and Spread The fungus overwinters in sunflower crop refuse above the soil surface. The disease is most severe under conditions of prolonged high temperatures and high humidity. Control The disease can be reduced by thorough discing in the fall to bury crop residue, and by crop rotation.
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