Sweet potatoes are a relatively easy garden plant to grow. They suffer from few serious diseases, making diagnosis of major problems simple. However, when the only symptom of sweet potatoes are white spots in their flesh or on their leaves, it may be trickier to determine the problem. Most causes of white spots do not require treatment and must be cured with a change in plant management.
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Fusarium Root Rot
Fusarium root rot is caused by the fusarium fungus. It is a serious problem for sweet potatoes in the South. Minor infections can appear as tubelike cavities in the sweet potato. A white, fungal growth often appears inside this tube. Fusarium can enter the root after mechanical injury, but susceptible slips planted in fusarium-infected soil are also affected. Do not plant sweet potatoes where fusarium problems have occurred in the past.
Edema is a noninfectious problem of sweet potatoes, which results from an imbalance in water uptake and transpiration. Roots that take in water too quickly lead to ruptured cells in the leaf dermis. In sweet potatoes, this damage manifests along leaf veins as white, crusty eruptions that resemble grains of salt. Feeding with additional calcium and potassium may help to strengthen cell walls if plants cannot be moved to a faster-draining location. Plants with mild edema tend to recover.
Nematodes can invade sweet potatoes when soils are still cool and moist. Root-feeding nematodes like ditylenchus destructor and lance nematode feed only on subterranean plant parts. This nematode class can move into the potatoes themselves and begin reproducing. Often, the foliage looks healthy until the plant is nearly dead. Peeling an infested sweet potato will reveal white to off-white spots in otherwise healthy tissue. Treating nematodes is tricky business, since some helpful nematodes feed on the plant parasitic nematodes. Using marigolds, sudan grass or brassicas as a green manure between crops is generally effective for mild nematode problems.
White spots on sweet potato leaves can be the result of the feeding activities of spider mites. Mites suck chlorophyll directly from plant cells, leaving small, white spots stippled across leaves. Unlike other causes of white spots in sweet potatoes, mites are easy to control. Hose plants regularly to knock mites off of leaves, being sure to spray the undersides of leaves. Horticultural oils can also be helpful if the infestation is severe.