Powdery mildew of parsnip

Symptoms

A dry, white-to-greyish powdery or floury coating develops on the upper and lower surfaces of the parsnip leaves and along the leaf stalks (petioles). The patches start as small, dust-like spots that spread and merge until whole leaflets look as if dusted with flour. Heavily affected foliage turns yellow, then brown, becomes brittle and dies back prematurely, and tiny black dots (chasmothecia) may appear within the older white felt late in the season.

Easily confused with

• parsnip canker

  How to tell them apart: Erysiphe heraclei (powdery mildew) coats the leaf as a dry, white-to-grey, dusty or floury film that can be wiped off with a finger, sitting on top of otherwise green tissue. Itersonilia perplexans (parsnip canker / leaf spot) instead produces discrete dark brown to black, often angular necrotic spots and blotches with a water-soaked margin that sit within the leaf and cannot be rubbed away, and it also causes the characteristic black-brown cankers on the root shoulder. In short: white removable dust on the surface points to powdery mildew, dark sunken fixed spots plus root canker point to Itersonilia.

• sclerotinia stem rot

  How to tell them apart: Both Erysiphe heraclei (powdery mildew) and Sclerotinia sclerotiorum (white mould / cottony rot) can show whitish growth on anise, but they are easy to tell apart. Powdery mildew is a DRY, dusty, superficial flour-like film on otherwise firm leaves and stems; it wipes off with a finger and leaves the tissue intact underneath. Sclerotinia is a WET, dense, cottony white mould at the stem base, crown or on collapsed stalks, accompanied by soft watery rot and — the clincher — hard black lumps the size of rat droppings (sclerotia) forming inside or on the rotted tissue. If the plant is wilting and rotting with cottony mould and black bodies, it is Sclerotinia, not powdery mildew.