Induced Resistance Against Plasmopara helianthi in Sunflower Plants by DL‐β‐Amino‐n‐butyric acid

Sunflower plants treated with the nonprotein amitio add, DL‐β‐attiino‐n‐butyric acid (BABA) were protected against infection with Plasmopara helianthi. Soil drenches at the highest rates (150‐250 mg/kg soil), applied one day before the inoculation induced high levels of protection (80‐83%) against the disease and more than 90% control was observed when BABA was applied at 300 mg/kg soil. However, at this concentration phytotoxic symptoms were observed. This compound also provided a curative activity when applied one day post‐inoculation. BABA had no antifungal activity in vitro against P. helianthi. The effect of BABA on zoosporangia germination was evaluated by treating pre‐germinated seeds with the compound solution and the zoosporangia suspension for 3 h. Then, seeds were sown and the percentages of infected plants were determined. The other two aminobutyric acid isomers (a and g) were ineffective against downy mildew. The mechanisms by which DL‐β‐amino‐n‐butyric acid protect sunflowers against downy mildew awaits more detailed elucidation.