The above-ground symptoms of this are nonspecific and include a general yellowing, wilting, and collapse of the foliage and the entire plant. Black lesions are sometimes visible on the stem at or near ground level.
Control can be difficult once plants are infected so prevention is important.
It is helpful to avoid overwatering, especially in heavy soils, and to avoid watering directly into the crown area of the plant. Highly symptomatic plants can be rogued and removed since recovery is unlikely. Downy mildew, Peronospora. Early symptoms of infection often go unnoticed and appear as slightly chlorotic areas on the upper leaf surface. Diagnostic symptoms develop as the fungus grows and breaks through the lower leaf surface and appears as a purplish-gray fuzzy growth.