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Yellowing leaves could meanYellowing leaves could mean several things, such as a fungus or virus, an insect pest (such as leafhoppers or aphids, which can spread viruses), or a cultural problem, such as heat or drought, or even overwatering. Make sure your plants have plenty of air circulation, good drainage, and only water when the soil feels dry down to about 1 inch below surface--the tubers can rot if overwatered. The soil should be dry, but not overly so. If a heat wave comes, try sheltering the plants with shade cloth or a screen of some kind. For fungus troubles, ask a garden nursery for the best control. For viruses, there is no cure, and the plants are best discarded. For leafhoppers or aphids, try applying insecticidal soap. For more advice about controls, you might check with a garden nursery.

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