Virus Diseases of Citrus Fruits

J. M. Wallace, T. J. Grant.

Psorosis designates a group of diseases of citrus that have certain characteristics in common, notably the symptoms they produce on young leaves. Among them are psorosis A, psorosis B, concave gum, blind pocket, crinkly leaf, and infectious variegation.

No one has been able to reproduce the diseases by isolating fungi or bacteria from diseased plants, but they have been transmitted by means of buds, leaves, and bark tissues. No insect vectors are known for them.

Studies in the field indicate that their spread is associated with man-made bud grafts and occasionally by naturally occurring grafts of root tissue. A characteristic symptom on young leaves of affected trees was discovered in 1933 by H. S. Fawcett, who demonstrated by further procedures that the diseases are caused by viruses.

Young-leaf symptoms appear during the growth flushes and vary greatly in extent and degree on individual trees at a given time and between seasonal peaks of greatest growth.

Small, elongated, cleared places, much lighter than the rest of the leaf blade, occur in the region of the vein-lets.

They may be numerous and scattered over the entire blade or they may occur on only certain parts of the leaf. At times most leaves of suitable growth show symptoms. At other times relatively few leaves have symptoms. It is not uncommon to find strong symptoms on one leaf and to find none on an adjacent leaf of the same age.

Often some of the small flecks are quite indistinct. Others coalesce to form large blotches. A characteristic pattern known as the zonate, or oak-leaf, pattern is sometimes present. In California that pattern occurs Or, nearly all leaves of the spring flush of trees affected with the concave gum psorosis. It is seldom found on leaves of later flushes.

The cleared areas gradually disappear as the leaves mature. Large soft leaves frequently show symptoms until they harden or become almost mature. The oak-leaf pattern generally is retained longer than the ordinary flecking or stippling symptom. A careful examination of both sides of the leaves is necessary to distinguish between faint psorosis symptoms and some effects caused by thrips, spider mites, or mechanical injuries. Symptoms are seen host readily when the leaf is shaded from direct sun and viewed with the transmitted light of the sky coming through the blade.

THE YOUNG-LEAF SYMPTOMS associated with concave gum and blind pocket types of psorosis are indistinguishable from those found on trees affected with the other types except that nearly all leaves of the spring flush of trees having concave gum show strong zonate patterns.

The outstanding features of concave gum psorosis are the concavities of various sizes that develop on the trunks and larger limbs. Usually a fairly normal bark covers the surface. Cracking of the bark often occurs in the central part of the concavity or around the margin and gum appears on the surface. Occasionally the bark may scale, but that may be due to the presence of psorosis A in the trees in addition to concave gum. The only symptom in the wood usually is the development of gum immediately under the concavity. As long as the concavities remain few, the trees appear not to be seriously handicapped, but many concavities may cause dwarfing and a decline in growth. Large limbs of affected trees are sometimes slightly twisted and irregular in shape although they do not have the typical concavities.

Blind pocket usually makes concavities deeper and narrower than those of concave gum. A smaller area of wood may be affected, so that the depressions may become very narrow or almost closed. In older lesions the wood under the central part of the blind pocket concavity shows more alteration than in the case of concave gum and is composed of a rather loose wood parenchyma, often impregnated with a waxy or gummy substance. The apparent point of origin is often found quite deep in the wood under the bottom of the pocket. Only rarely is gum exuded to the surface.

The concavities formed on trees affected with either concave gum or blind pocket psorosis cannot always be defined as being that of either one or the other of these types of psorosis. Most of the differences might be explained on the basis of varietal response or growth rate of the affected trees. On citrus varieties such as mandarin (which often produces uneven, fluted, or grooved trunk growth) it is not easy to diagnose the diseases with certainty or to tell which type is present. From a practical standpoint !his latter point is of no particular Importance. The absence of the zonate pattern on the young leaves of trees suffering from blind pocket indicates that the two types of psorosis are distinct even though they are very probably caused by closely related virus strains.

CRINKLY LEAF, seen mainly on lemons, induces young-leaf symptoms Much like those of the other psorosis types. Some of the leaves also become Pocketed and warped. Carried to sweet orange, it causes typical young- leaf symptoms and some puckering or Cupping of leaves. Sweet orange root-stocks of lemon tops affected by crinkly leaf develop bark symptoms typical of psorosis A. Crinkly leaf disease may result from a mixture of two viruses, one of which is the psorosis A virus and the other a yet unidentified virus. Crinkly leaf may even be a strain of psorosis A. Experimental inoculations with all possible combinations of the other known psorosis types have not produced the crinkly leaf disease. Crinkly leaf is not found as commonly as psorosis A, possibly because the leaf symptoms are so striking on lemon that propagation from diseased trees has been easily avoided by informed nurserymen.

Infectious variegation psorosis is found occasionally on lemons, but is not very important. It has been transmitted by means of buds. On young leaves of lemon, orange, and other citrus it causes symptoms like those of the other psorosis types. On some of the older leaves of lemon, grapefruit, and sour orange, parts of the blades are white or pale yellow. Sometimes the chlorotic or variegated areas are entirely on one side of the midrib. Sometimes they are scattered over the entire blade in no definite pattern. Some leaves near affected ones appear normal. The white or light-yellow color persists on affected leaves and does not disappear, as in the case of the flecking symptom of young leaves. There may also be rather marked distortion of leaves. Fruits on affected lemon trees are sometimes small, misshapen, and bumpy. Some of the symptoms of crinkly leaf are usually associated with infectious variegation. It has not been determined whether infectious variegation is caused by a mixture of virus strains, nor has it been possible to produce the symptoms of infectious variegation by infecting citrus trees with various combinations of the other types of psorosis.

PSOROSIS A and psorosis B cause the bark condition commonly known in California as scaly bark. The psorosis types that cause bark lesions occur the world over and should not be confused with leprosis (which is called scaly bark in Florida, where psorosis also occurs). Sweet orange, grapefruit, and tangerine trees infected with psorosis A or psorosis B commonly develop the typical bark lesions, but lemon and sour orange never show bark symptoms.

Psorosis A and psorosis B seem to be caused by closely related virus strains, differing perhaps only in virulence. Psorosis B produces more harmful effects; besides the young-leaf and bark symptoms, it induces symptoms on old leaves, twigs, and fruits. The persistent symptoms on older or mature leaves consist of circular spots or rings of varying sizes, usually comprised of a yellowish-green central part surrounded by a light-yellow border. As the leaves mature, the under side of the affected area becomes yellowish brown and slightly raised, as though gum had formed in the leaf tissues. Some fruits of trees affected by psorosis B show surface rings bordered by sunken grooves of different sizes and patterns. Young green fruit sometimes has round spots like those on mature leaves. In other cases, especially on grapefruit, large circular grooves, partial rings, or irregular circles occur on the rind and result in a rough, bumpy condition.

On trees infected with psorosis B, the bark of green twigs and small limbs often shows raised, corky areas like those on mature leaves. As the twigs become older, the bark develops reddish-brown areas that split and flake off.

Psorosis B is not often encountered in the field. Because affected trees grow poorly and show striking symptoms early, propagators usually are able to avoid trees with psorosis B.