What Is It? | Facts in Depth | For the Professional Diagnostician
Brassica Diseases | Downy Mildew
Downy Mildew
Symptoms
Plants can be infected at any stage of development. In seed beds, cotyledons and primary leaves are invaded first resulting in fungal growth visible on the underside of the leaf, while chlorosis develops on the upper side of the leaf. The young leaves or cotyledons, when severely chlorotic, may drop off prematurely. Older leaves usually persist and infected areas gradually enlarge, turn bright yellow, and then become tan and papery. In rare cases, the infected leaves may develop hundreds of minute darkened specks. Under cool, moist conditions, a white mildew forms on the undersides of infected leaf lesions.
Disease symptoms may appear on other plant parts as well. Turnips and radishes may develop an internal, irregularly shaped discoloration extending from the crown downward. The flesh may be brown or black or show a form of net necrosis. In advanced stages, the skin becomes roughened by minute cracks and the root may split open. Dark grey-black, sunken spots develop on cauliflower curds and cabbage heads, often followed by secondary invasion by soft-rotting bacteria and fungi. The disease may progress systemically through the plant, resulting in dark brown to black streaks in the veins.
Downy mildew on cauliflower (left) and radish (center; right).
Signs
When environmental conditions are conducive, a white mycelium grows on the undersides of infected leaf lesions. Within this growth are numerous branched sporangiophores with slender curved tips. Sporangiophores with a length of 115 to 410 μm (average 295 μm) end with sterigmata bearing single sporangia. Sporangia are ovoid and measure 18 to 28 × 25 to 45 μm (average 22 × 35 μm).
Often Confused With
- Rhizoctonia on turnip and radish – Rhizoctonia causes internal decay and necrotic lesions to turnip and radish similar to that caused by downy mildew; however, on turnip the decay is much more watery than the necrosis caused by downy mildew. On radish, Rhizoctonia symptoms are more superficial than those caused by downy mildew, although surface cracking is common for both diseases.
- White rust – White rust is caused by Albugo candida and causes similar symptoms to those caused by downy mildew. There is a white grown on the undersides of the leaves which can be confused with downy mildew; however, the upper sides of the leaves have different symptoms. White rust causes pustules while downy mildew causes chlorotic lesions.
Isolation Media
Peronospora parasitica is an obligate biotroph and therefore cannot be cultured on laboratory media.
Rapid Diagnostic Tests
- Internal primers were designed for specific amplification of full ITS and ITS2 regions of parasitica: PpITS1F (5’-CAAYTWTAATTGGGGG TCGTGATCTT-3’), PpITS2F (5’-AAGCGTGACG ATACTAATTTG-3’) and PpITS2R (5’-TGAAGTG CGGCCGAAGCTT-3’).