When early-season southern winds bring leafhoppers that are infected with the aster yellows phytoplasma (Candidatus Phytoplasma asteris) up from the United States, and conditions in a field are favourable for leafhoppers, watch for aster yellows disease. It is often first spotted in plants that are noticeably taller than the rest of the crop (through flowering to podding stages). Infected plants turn a blue-green colour and leaves will show a red or purple tinge late in the season. Sterile, green, leaf-like structures will replace the flower and round or oval, blue-green, hollow, flattened bladder-like structures replace seed-bearing pods in infected plants. Although there have been instances when economic thresholds have been reached, such as in outbreak years, generally few fields reach an economic level of loss.
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