Induced Resistance In Maize Against Curvularia Leaf Spot Disease By Exogenous Chemicals

The maize curvularia leaf spot caused by Curvularia. lunata (wakker) Boed was an important epidemic disease in recent years. Effects of 9 exogenous chemical compounds on induced resistance to curvularia leaf spot and physiological mechanism induced by salicylic acid (SA) were studied.The tested chemicals, at concentrations necessary for resistance induction, did not directly affect growth oi' the pathogens.The induced resistance was detected after maize being treated with these inducers and challenge inoculated with Curvularia.lunata. Results showed that 5 chemicals were effective to induce resistance. Among them, SA was the most effective and reduced disease severity, relative to controls, by up to 49.6% and the effects of riboflavin, vitamin K₃, H₂C₂O₄ and K₂HPO₄ were 38.8%, 41.5%, 27.2%, 37.8%, respectively. The high susceptible cultivar Chandan13 and the susceptible cultivar Tangshan both responded with reduced development of maize curvularia leaf spot (C.lunata) when they were treated with chemicals, but the effect of induction resistance in susceptible cultivars was higher. Moreover, the induction resistance was more effective in seedling stage than in jointing stage and booting stage.The optimum concentration of SA treatment to induce resistance of maize seedlings was 10mmol/L, which showed requirement of induction with high concentration of salicylic acid from Curvularia.lunata. The highest level of induced resistance was acquired when maize leaves were sprayed with 10mmol/L SA and inoculated with fungal spore suspension three days later. In SA-treated maize seedlings , induced resistance was demonstrated to be lasted for 10¹5 days. When the leaves were treated with 0.5mmol/L riboflavin or 0.1 mmol/L vitamin K₃ would also lead to reduce of disease severity, the effects were 36.73%, 29.08%, respectively. Resistance to infection was more effective by delaying inoculation for up to 2 days following riboflavin application. The induced Curvularia leaf spot resistance of the riboflavin-treated seedlings was observed to be last for about 7 days. Furthermore, when the lower three leaves were sprayed with 10mmol/L salicylic acid, 0.5mmol/L riboflavin or 0.1 mmol/L vitamin K₃ before challenge inoculation, systemic acquired resistance on the unspayed upper leaves were observed. Results showed that resistance of maize by exogenous chemicals were typically a systemic acquired resistance (SAR) characterized by systemicity and durability. Results that were also conducted suggested that endogenous mobile signaling mechanism was elicited andfunctioned between treated leaves and their upper non-elicitor-treated ones.SA was screened for the ability to induce enzyme activity in maize leaves. After maize seedling leaves were sprayed with lOmmol/L SA, the activities of phenylalanine amonnia lyase (PAL) ,peroxidase (POD) and polyphenol oxidase (PPO) increased significantly compared with those in water controlled leaves, and several new isozymes of POD were detected, which showed three new isozyme in high susceptible maize cultivar (Chuandanl3) and five in susceptible maize cultivar (Tangshan). But the change of catalase (CAT) activity after treatment was different that showed a slight transient decline, which were suppressed after SA-treated. Meanwhile, increment of lignin content was also observed in SA-treated leaves. The content of malonaldehyde (MDA) was declined along with the day after treating with SA. Time courses of these indices coincide with the expression of SA-induced resistance exactly.Results mentioned above indicated that the elevated level of PAL and POD were positively correlated with the enhancement in disease resistance in maize induced by SA that we reported previously. And the change of PAL and POD activity were accord with lignin content, which provide direct evidence for they can enhance secondary product synthesis of lignin and acting as key enzyme in plant defense response. Therefore, the remarkable increase of the enzyme in plants treated with salicylic acid and then inoculated with Cwvularia.hinata, which evidently revealed that the decrease in disease severity following treatment with these agents was a result of resistance induction.