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 Septoria leaf blight
• pistachio leaf blight
• 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.