Snapdragon

Seedlings and young transplants suddenly wilt, topple over and die, often in patches. At the base of the stem, just above the soil surface, the root collar turns black, softens and constricts, so the plant loses its support and falls over. Seeds may also rot before emergence, which shows as sparse, uneven emergence of vegetable transplants, annual flowers and herbs raised from seed. Two phases are distinguished: pre-emergence damping-off (rotting of the germinating seeds, absence or gaps in emergence) and post-emergence damping-off (toppling of seedlings that have already emerged). The exact picture depends on the pathogen. With infection by oomycetes (Pythium) the rot is watery, translucent and soft — the constricted stem looks as if 'cut with a knife', and the seedling falls flat, especially in cold and excess water. Rhizoctonia solani produces a brown to rusty-red constriction, the tissue is drier and more pinched than watery, the seedling may remain standing for some time, and a delicate, brown, cobweb-like mycelium is sometimes visible on the surface of the medium and on the collar. Fusarium causes wilting with yellowing and discoloration at the base and of the roots. The focus spreads in a circular, patchy way from the first infected seedling, because the pathogen travels through the moist medium and water; the roots of diseased seedlings are brown, poorly developed, with sloughing bark.

Rusty-orange to brown, powdery clusters of spores on the underside of leaves, matched on the upper side by yellow spots. Heavily infected leaves turn yellow and drop prematurely, and the plant weakens. It affects roses and mallows among others, and some rusts have an alternate host (for example pear rust and juniper). The pustules (clusters of summer spores, the so-called uredinia) break through the leaf skin and release an orange-rust powder that rubs off onto the finger - this is the most reliable identifying feature. In high summer they appear mainly on the underside of the blade, along the veins and on the leaf stalks, and in some species also on green shoots and sepals. In late autumn the clusters darken to brownish-black - these are the overwintering spores (telia) by which the fungus survives winter on plant debris. Rusts are highly host-specific: a given rust usually infects only one species or genus of host, which is why the rusts of rose, mallow and marsh mallow, bean, leek and garlic, mint, pelargonium or lawn are different, non-interchangeable fungi. The general symptom - yellowing and premature leaf drop - weakens the plant and limits flowering and yield, though it rarely kills it within a single season.