Fraser photinia

Blossoms and young shoots suddenly turn brown and then black, looking as if scorched by fire — hence the name of the disease. The tips of infected shoots bend downwards into a characteristic hook resembling a shepherd's crook. In humid conditions a milky, sticky bacterial ooze seeps from the infected tissue, and the infection spreads rapidly down the shoot, killing whole branches. Infected leaves and fruit dry out but remain attached to the tree for a long time. Pear and quince are the most severely affected, followed by apple, and among ornamental and wild plants — hawthorn, cotoneaster, firethorn (Pyracantha) and rowan, which can be a dangerous reservoir of infection. On the bark of the branches and trunk, sharply delimited, sunken necroses (bark canker) form with cracking bark; when the bark is cut, the tissue beneath is discoloured reddish-brown, often with marbled streaks. The drop of bacterial ooze eventually turns amber and dries, and the presence of the bacterium can be confirmed by the so-called streaming test — from freshly cut infected tissue immersed in water, threads of a cloudy bacterial suspension flow out. Infected fruit turns black and mummifies, remaining on the tree, which distinguishes fire blight from ordinary wilting caused by water shortage.

Small (1–3 mm) soft-bodied insects, green, black or pink, feeding in clusters on young shoots and the underside of leaves. They excrete sticky honeydew. The diagnostic feature that distinguishes aphids from other small sucking insects is the pair of siphunculi (tubular projections) at the rear of the abdomen together with a tail-like cauda; a single colony usually holds wingless forms (parthenogenetic females) alongside winged migrants that fly onto new plants in summer. In summer aphids reproduce parthenogenetically and viviparously over many generations, so the population builds up explosively; in autumn many species produce a sexual generation that lays overwintering eggs on the host, and host-alternating species move between a winter host (often a tree or shrub) and a summer host (a herbaceous plant). Feeding symptoms include curling, crinkling and distortion of young leaves, stunted growth, yellowing, and the shedding of buds and fruitlets. Abundant honeydew coats the leaves, and the sooty mould fungus that develops on it forms a black film which limits photosynthesis; the honeydew is fed on by ants, which in return defend the colony against predators. Aphids are also important vectors of plant viruses (mosaics among them), transmitting them from plant to plant even during a brief probing puncture.