Rust

Symptoms

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.

Treatment

Remove and destroy infected leaves and improve air circulation. Apply fungicides in cases of heavy infection. With host-alternating rusts it is worth avoiding the proximity of alternate hosts. Pick off infected leaves as soon as the first pustules appear and remove them from the area (do not compost - dispose of the debris with household waste or take it to a municipal waste collection point), and cut out heavily affected shoots. Limit the wetting of leaves and switch to watering at the base, because water on the blade favours spore germination. Chemical intervention is protective and suppressive in nature - it will not cure tissue that is already damaged; its aim is to protect healthy, young leaves. At the first symptoms use sulphur products (contact) or systemic fungicides (triazoles), and under heavy pressure the treatments are repeated at intervals in line with the label; to avoid resistance it is worth rotating products with different modes of action (for example triazoles with strobilurins). In host-alternating rusts (such as pear-juniper rust) lasting improvement comes from removing the alternate host from the immediate vicinity, where this is feasible.

Prevention

An airy site, watering at the base, avoiding wetting the leaves, choosing resistant varieties, and raking up and removing fallen leaves in autumn. Plant at an appropriate spacing and thin out crowded plantings so that the leaves dry quickly and air circulates freely. Water in the morning and at the base of the plant. Feed in a balanced way - avoid an excess of nitrogen, which produces soft, susceptible growth, and provide potassium to strengthen the tissues. In autumn rake up and remove fallen leaves and debris thoroughly, as the fungus overwinters on them. In vegetable crops prone to rust (leek, garlic, bean) practise crop rotation and do not grow the same species in the same spot year after year. Where possible, separate the plant from alternate hosts (for example junipers near a pear tree) and choose cultivars with increased resistance. During warm, humid periods carry out regular inspections of the underside of the leaves to catch the first pustules before the rust spreads.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.