Common juniper

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.

The disease has a two-host cycle: the pear is the main host, while juniper — especially savin and Chinese juniper, on which the fungus overwinters — is the alternate host. In spring, bright orange-red spots appear on the upper surface of pear leaves and gradually enlarge. In summer, warty, sieve-like outgrowths (aecia) that release spores develop on the underside of the leaves, opposite the spots. On juniper the fungus forms spindle-shaped swellings on the shoots, which in spring swell into gelatinous, orange outgrowths in wet weather. The spots on the upper leaf surface are initially small and yellow-green, later turning a bright orange-red with a reddish rim; at their centre, on the upper side, tiny black dots (spermogonia) are sometimes visible. The corresponding aecia on the underside take the form of light-brown, tube-like sieve-shaped outgrowths a few millimetres long, gathered in clusters — it is precisely this lattice-like arrangement that gave the disease its German name 'Birnengitterrost' (pear lattice rust). Under heavy infection there are many spots, the leaves yellow and drop prematurely, the tree weakens and yields less; occasionally spots also appear on the leaf stalks, young shoots and fruit. The cycle is strictly one-directional: in spring, in wet weather, the gelatinous outgrowths on juniper release basidiospores carried by the wind even hundreds of metres, which infect only the pear — the fungus does not spread from pear to pear, while the mature aecia on pear leaves in turn release spores that infect the juniper again, not other pears.