Biological and Urea Treatments Reduce the Primary Inoculum of Red Leaf Blotch of Almond Caused by Polystigma amygdalinum

Red leaf blotch (RLB), caused by Polystigma amygdalinum, is considered the most prevalent foliar disease in both traditional and new intensive almond-growing areas in Spain. Since the disease is monocyclic, its control must be based on the reduction of the only source of inoculum—the leaves infected in the previous season and fallen to the ground in autumn. Thus, this study aimed to determine the effect of two microorganisms and urea on RLB inoculum reduction by evaluating different application modes to fallen leaves in field conditions. Leaves of almond cv. Guara showing symptoms of RLB were collected in autumn, placed into nylon mesh bags, and treated by dipping or spraying with conidial suspensions of Myrothecium inundatum or the nonpathogenic strain Fusarium oxysporum FO12. The bags were exposed on the ground or buried in an experimental almond field for six months in each experimental year. Bags treated with crystalline urea solution at 46% N or not treated were included as controls. The primary inoculum (number of ascospores per gram of leaf) and the development of fruiting bodies (maturity stages of perithecia) were monitored in the fallen leaves for each experimental treatment combination. Myrothecium inundatum significantly reduced the primary inoculum in comparison with the nontreated control or F. oxysporum FO12, showing a similar effect to that observed for urea in the two experimental years. The type of application (spraying or dipping) did not show any significant effect, whereas the inoculum was significantly reduced in buried leaves in comparison to leaves maintained on the ground for all the treatments tested. This study represents the first report evaluating management strategies against RLB based on the reduction of the primary inoculum of P. amygdalinum.