Aster yellows of wheat
Aster yellows is caused by a phytoplasma (Candidatus Phytoplasma asteris) spread by leafhoppers during feeding. Leafhopper feeding can cause white flecking or stippling on leaves. When the plant is infected, leaves become yellowish and progressively turn red-brown or purple, usually starting at the tips. Discoloration typically occurs as streaks. Plants may be stunted and excessively branched, and yields may be reduced. Leaves, stems, and heads may become curled and twisted. Symptoms tend to occur along field edges in patches and are often mistaken for other diseases such as barley yellow dwarf or nutrient deficiencies. Symptoms in wheat are often correlated with those in canola or other dicots and weeds in the same season, and can appear at anytime during the growing season.
Conditions that favor the leafhopper vector will favor aster yellows development. Leafhoppers generally migrate from southern states, many of them already carrying the phytoplasma. Leafhoppers take flight when temperatures exceed 59°F and cooler temperatures or rain will stop their migration until weather conditions become more favorable. After feeding and transmission of the pathogen, plant symptoms appear in 2-3 weeks. Wheat plants are most susceptible during early vegetative growth stages and extent of damage depends largely on when infection occurs. Symptoms can be more pronounced under stress conditions such as wet or waterlogged soils.
Resistance has been identified in a few varieties of spring wheat. Applying insecticide to manage leafhoppers is not recommended since leafhoppers may continue to migrate into the field after application. Manage weeds to reduce the number of plants that may serve as sources of inoculum.