European pear rust

Symptoms

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.

Treatment

Cutting out and destroying the infected, swollen juniper shoots reduces the source of infection for pears. In spring, during the infection period, pears can be protected with fungicide sprays repeated while spores are being released from the juniper. Infected pear leaves should be removed, although this does not break the disease cycle, because the fungus does not overwinter on the pear. Effectiveness depends above all on reducing the source of infection, that is the juniper — spraying the pear alone, without removing or distancing the infected junipers from the surroundings (even up to several hundred metres), gives only partial results. The swollen juniper shoots are best cut below the swelling in late winter or early spring, before the spring moisture swells the gelatinous outgrowths and releases the spores; the cut material must be burned or carried away. Fungicidal protection of the pear is carried out preventively during the infection period — from the bud-break and flowering stage until about June — repeating treatments every 10–14 days in wet weather, so as to hit the moment when spores are released from the juniper; once the spots have formed, intervention is much less effective. Removing infected pear leaves does not break the cycle (the infection returns from the juniper anyway), but it reduces the number of aecia releasing spores that infect the juniper, so applied consistently over the years it lowers disease pressure in the surroundings.

Prevention

The key is spatial separation of the two hosts — pears should not be planted near savin and Chinese junipers, and in areas of high disease pressure it is best to give up these juniper species in favour of resistant cultivars. Choosing less-susceptible pear cultivars also helps. Because the spores travel with the wind for hundreds of metres, full isolation is sometimes unattainable in dense housing — it is then worth removing susceptible junipers from your own plot and encouraging neighbours to do the same, and monitoring the trees in spring. The most susceptible are savin juniper (Juniperus sabina) and Chinese juniper (J. chinensis) and their cultivars; common juniper (J. communis) is usually resistant or little susceptible, so instead of giving up junipers entirely you can choose less-sensitive species and cultivars. In late winter it is worth inspecting the junipers and cutting out the spindle-shaped swellings before they swell. On the pear side the following help: an airy site and thinning cuts (rapid drying of the leaves), balanced fertilisation, autumn raking and removal of fallen leaves, and the selection of less-susceptible cultivars. The disease rarely threatens the tree's life, so for isolated spots sanitary measures are usually sufficient without resorting to chemicals.

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Eliminating the juniper — the alternate host	Removing or distancing susceptible junipers (savin, Chinese) reduces the local spore source — the fungus overwinters on juniper, not on the pear.	Late winter / early spring, before the gelatinous outgrowths swell
Sanitary pruning and removal of infected organs	Cutting out swollen juniper shoots and collecting infected pear leaves reduces the spore mass; it acts as a support measure when the juniper stays nearby.	Juniper — early spring; pear leaves — throughout the season
Cultivation practices and less-susceptible cultivars	An airy site and thinning cuts speed up leaf drying, while choosing less-susceptible pear and juniper cultivars lowers disease pressure.	—

Chemical methods

Method	How it works	When to use
Systemic fungicides (triazoles, DMI/SBI)	Absorbed products that inhibit the fungus's sterol biosynthesis; they protect young pear leaves, applied preventively every 10–14 days in wet weather.	From bud-break and flowering until about June
Rotation with a fungicide of a different mode of action (strobilurins, QoI)	Alternating use or mixtures with a fungicide of a different mode of action reduces the risk of the pathogen developing resistance in the spray programme.	—

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.