pistachio rust

Symptoms

Yellow then rusty-brown pustules on the underside of leaves, premature defoliation by midsummer, reduced nut filling and yield, repeated infections weaken the tree.

Easily confused with

• pistachio twig blight

  How to tell them apart: Pileolaria terebinthi (pistachio rust) shows reddish-brown to violet, somewhat angular spots with a yellowish halo, mostly on the LOWER leaf surface; as it matures these become rough, crusty, scab-like dark-brown telia and the leaf curls or corrugates and drops early. Pestalotiopsis pistaciae instead makes greyish-brown necrotic blotches, often with concentric zoning and a darker margin, dotted with tiny BLACK pinhead acervuli that ooze a dark slimy spore mass when wet, and it commonly runs back into the shoot causing tip dieback. So: orange-to-rusty angular spots and crusty stalked telia on the leaf underside point to the rust, whereas grey zoned blotches with black specks plus shoot dieback point to Pestalotiopsis.

• olive Verticillium wilt

  How to tell them apart: On Pistacia terebinthus both Pileolaria terebinthi (pistachio rust) and Verticillium dahliae (Verticillium wilt) start as diffuse interveinal yellowing, scorch and premature leaf drop, but the decisive signs differ. Pileolaria terebinthi forms small round-to-irregular reddish-brown or purplish pustules ringed by a yellow halo on the leaf, petiole and fruit; turn the leaf over and you find raised powdery rust spore masses on the underside, while the wood stays clean when cut. Verticillium dahliae shows NO pustules: instead one or a few branches wilt and die back (often one-sided), and slicing a wilting twig reveals brown to greyish streaks or rings in the vascular wood, with symptoms worst in hot, dry spells.

• pistachio Septoria leaf blight

  How to tell them apart: Septoria leaf spot caused by Mycosphaerella pistacina shows small, sharply defined, angular brown-to-black spots on the upper leaf surface, typically 1-3 mm and often bounded by leaf veins. Tiny black dots (pycnidia) sit within the older spots and can be seen with a hand lens. Spots stay flat and do not raise into pustules; heavy infection causes general yellowing and premature leaf drop in humid late-summer conditions. Pistachio rust caused by Pileolaria terebinthi begins as faint chlorotic or pale necrotic flecks, but soon erupts into raised orange to yellow-brown pustules (uredinia) mainly on the lower leaf surface that release rust-coloured powdery spores onto a finger. Later in the season these darken to chestnut-brown to black telia. The powdery, erupting, raised texture and spore rub-off are the decisive signs absent in flat Septoria spots.

• pistachio leaf blight

  How to tell them apart: Pistacia rust begins as diffuse yellow-to-orange chlorotic flecks, but the decisive sign appears on the LOWER leaf surface: raised pustules that rupture to release rusty orange-brown powdery spores which smear onto your finger. Later in the season the pustules darken to chocolate-brown crusty telia. Unlike Septoria, the lesions are rounded, blister-like and powdery, not flat angular vein-bounded spots. Septoria leaf spot shows small (1-3 mm), sharply angular brown-to-grey lesions on the upper leaf surface, often bounded by leaf veins so they look box-like. Hold the leaf to the light: inside the dead spots you see tiny black dots (pycnidia), the fungus's fruiting bodies. Spots stay flat and dry, do not produce powder, and tend to coalesce into larger necrotic blotches in wet, mild weather.