Sunflower Production - NDSU Agriculture - North Dakota State ...

■ Charcoal RotDescription: Charcoal rot is caused by Macrophominaphaseolina, a fungus that attacks about 400plant species, including sunflower, dry bean, soybean,corn and sorghum. Charcoal rot is found throughoutthe Great Plains, but the disease is most common andsevere in southern areas such as Texas, Kansas andNebraska. Charcoal rot has been found on sunflowerin western North Dakota and north-central SouthDakota recently, and also on corn and soybean in bothstates. Charcoal rot generally appears after floweringbut seedling blights have been reported. Symptoms onstalks appear as silver-gray lesions near the soil line(Figure 93), which eventually decay the stem and taproot, leaving a shredded appearance. Stems are hollowand rotted, and lodge easily. Plants show poor seed filland undersized heads. Seed yield, test weight and oilconcentration are reduced. Numerous tiny black fungusbodies, called microsclerotia, form on the outsideof the stalk and in the pith. To the unaided eye, the microsclerotialook like pepper grains; with a 5X to 10Xlens they are clearly distinguishable as black, sphericalsclerotia. Another unique characteristic of charcoal rotis the compressing of pith tissue into horizontal layers,like a stack of separated coins (Figure 94). This is adiagnostic characteristic of the disease.Disease Cycle: The primary source of inoculum issclerotia in the soil, but Macrophomina also can beseed-borne. Upon stimulation by nearby root exudates,the sclerotia germinate to form mycelium thatcolonizes the roots. Macrophomina may colonizeroots early in the season, but disease symptoms donot manifest themselves until anthesis. Once the rootsystem is colonized, the fungus enters the stem andcolonizes the vascular system, resulting in wilt andpartial degradation of the pith. Disease developmentis favored by soil temperatures above 85 F. Moisturestress during the post-flowering period greatly favorsdisease development.Damage: Post-flowering stresses due to high plantpopulation or drought coupled with heavy applicationsof nitrogen fertilizer, hail or insect damage promotedisease development and accentuate the impact ofcharcoal rot. Yield losses can be significant if diseaseincidence is high, as infected plants die before seed setis complete.Management: Crop rotation, balanced fertilizer programsand practices to reduce moisture stress all helpminimize the impact of charcoal rot. Certain hybridsoffer some resistance, possibly through drought tolerance.Since the fungus also attacks corn, sorghum andsoybeans, not growing sunflower and these crops insuccessive years on the same ground would be advisableif charcoal rot has been observed.■ Figure 93. Silver grey discoloration of lower stemcaused by charcoal rot compared with healthy greenstalk (left). (T. Gulya)72■ Figure 94. Charcoal rot affected stalk split apart toreveal characteristic compression of pith into layers.(T. Gulya)