The branch wilt of Persian walnut trees and its cause

Abstract does not appear. First page follows.

Introduction

A branch wilt disease affecting Persian walnuts (Juglans regia L.) in California has been reported (Wilson, 1945), (1946).3 The trouble was first noticed about ten years ago in the southern San Joaquin Valley; but, according to some growers, it has been present in the Sacramento Valley for an equal length of time. Within the past five or six years it has become a major disease of certain walnut varieties throughout the central valleys.

The Disease

Name. Growers call the disease “wilt,” “limb wilt,” “twig wilt,” or “branch wilt.” The fourth name seems preferable, since the outer branches, not the limbs or twigs, are most commonly affected and since the disease is not a wilting of the entire tree, as the term “wilt” might imply.

Symptoms. As would be inferred from the name, the principal manifestation of the malady is the wilting of certain branches. During July and August the leaves wither, turn a deep brown, dry up (plate 1, A, page 443), but remain attached to the twigs. Because wilted leaves are very conspicuous against the surrounding green foliage, the disease is easily detected in an orchard during late summer. Only one other malady that might be confused with the branch wilt has been encountered. This is a twig blight4 occurring in summer and suddenly withering the leaves on twigs; but it seldom if ever kills large branches in one season. Specimens of this other disease on small branches 1.0 to 1.5 inches in diameter were received from southern California; but in this instance evidently the disease first attacked a twig and then gradually extended to the branch, which it killed only after two or three years.

Once established in the outer branches, the branch wilt proceeds into the limbs (plate 1, B) and within a few years may involve most of the treetop.