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PEONIES
The Manual of the American Peony Society
EDITED BY JAMES BOYD
Copyright 1928 by American Peony Society
FUNGOUS DISEASES AND PESTS OF THE PEONY
By Freeman Weiss, B.S., ph.d.
editor's note.—Dr. Freeman Weiss was born in Minneapolis, Minnesota, in 1892. He received the degree of Bachelor of Science from the University of Minnesota in 1915 and the degree of Doctor of Philosophy from Cornell in 1923. Dr. Weiss has spent three years as Research Assistant in Horticulture and Assistant in Plant Physiology at the University of Minnesota, one year as Assistant in Botany at Cornell University, and seven years as an Assistant and Associate Pathologist in the United States Department of Agriculture. He is now in charge of work on diseases of herbaceous ornamental plants conducted by the Bureau of Plant Industry of the United States Department of Agriculture.
IT IS probably the general experience of amateur peony-growers (using the term "amateur" more in the sense of inexpert than in reference to vocation) that failure to bloom is the most common and annoying trouble to which the peony is heir. Botrytis blight, which attacks buds and foliage, and the other leaf-spots are readily recognized as diseased conditions, to ameliorate which one can proceed with some confidence to apply simple and specific control measures. But when a plant persistently makes a weak growth in the absence of any apparent cause, or produces only small buds which soon abort although the foliage-growth seems thrifty, the condition is much more difficult to diagnose, and often still more so to remedy.
Failure to Bloom.
Accordingly, the various factors known to predispose a plant to weak, shy, or no blooming will be listed and discussed first. In so far as these are primarily factors of culture relating to site, soil, manner of planting, fertilization, mulching, etc., the remedy is to be sought in the sections of this Manual which deal with peony culture, but if the condition is due to a disease, the appropriate remedy, so far as known, will be given below under the heading of one or another of the different diseases.
Failure of peonies to bloom may result from:
(i) An unfavorable site. There may be excessive shade, when peonies are planted in lawns under or near large trees, or too close to tall shrubbery. In these circumstances there is usually a concomitant deficiency of soil-moisture.
(167)
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(2) A hard or impervious subsoil. In wet weather this results in the topsoil becoming soggy and poorly aerated; in dry weather it interferes with the capillary rise of moisture from deeper levels; at all times it restricts root-growth and the feeding-area upon which the plant depends for water and mineral nutrients, with a corresponding reduction in the size of the top and the amount of bloom.
(3) Too deep planting. The crown, or juncture of the aerial shoots with the large, fleshy roots, should be just below the soil-surface. If the crown is buried several inches, a gnarly and much-branched growth of stem develops between the roots and the shoots. Plants having this type of crown are especially predisposed to decay or to develop galled overgrowths, and often there is excessive production of shoot-buds so that none of them develop vigorously.
(4) Recent transplantation, especially of divisions from decayed, galled, or otherwise injured roots, more so if the divisions are very small.
(5) A diseased condition of the root. This includes root-decay and root-knot or root-gall; also crown-gall, and perhaps certain hereditary diseased conditions.
(6) Attack of shoots or buds by Botrytis blight.
Botrytis Blight.
This disease has been reported to occur throughout the geographic range of peony-culture. Frequently i't is epidemic in wet springs and may destroy the bloom in entire plantings. It affects the stems, buds, and leaves, beginning in the spring as the shoots emerge. Infected stems show a dark-colored area at the base or just below the soil-surface. The discolored and injured portion may be shallow, so that the foliage appears but little affected, though the buds are blasted, or the infection may extend so deeply that the leaves and stem blacken, decay, and die. The rotted parts become covered with a brown, felt-like coat of fungus-spores which are borne in minute heads on tiny stalks. These spores are spattered by rain, or carried by air-currents and by insects to other parts of the plant, particularly the buds, where the sugary exudation furnishes an ideal medium for lodgment and germination. As a result, one bud after another may become infected. Buds attacked at an early stage turn black and dry up; larger buds turn brown and fail to open.
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Often the flower-stalk is rotted off. Some buds nearly reach the flower stage before succumbing, or open flowers may be attacked, resulting in deformed, undersized blooms with a brown, decayed center. The leaves may be more or less generally affected with irregular brown spots, or on some of the shoots are killed outright. The severity of the attack is influenced greatly by the wetness of the weather; and the stage of a generalized attack following blighting of the buds may not appear in a dry season. In wet seasons the stem-rot may extend into the crown and prepare the way for root-decay.
This blight is caused by parasitic fungi of the genus Botrytis, the gray mold. An early and a late form of the blight are attributed to different species, but for the purpose of this discussion may be treated as one. The fungus lives over winter in the form of small, black, pimple-like sclerotia or resting bodies, attached to dead stems or decayed portions of the crown, by disintegration of which they pass into the soil. Their germination in the spring gives rise to a crop of spores which may infect the emerging peony shoots, no form of injury being necessary to prepare the way. However, a mulch about the bases of the stems at this time aids infection by holding moisture, even though the material used in mulching is not itself infested.
No important differences in the susceptibility of different varieties of Botrytis blight have been noted. In Whetzel's tests, spraying with Bordeaux mixture proved ineffective in preventing the bud-rot form of this disease, and it was found that the toxic action of copper, which gives Bordeaux its fungi-cidal value, was neutralized in the sugary exudate from peony buds. On the other hand, the experience of growers testifies that spraying is of value in reducing the amount of shoot and leaf-blight, and Whetzel now reports* that a copper-lime dust is quite as effective for this purpose as Bordeaux and does not cause spotting of the foliage. If applications of dust are begun early in the season, and are made at timely intervals, just before a rain, or when the plants are wet with dew, there results a marked reduction of blight on young shoots, and also much less bud-rot since there are fewer sources of disease from which the buds ^become infected.
The fungi which cause this blight are very widely distributed and there is no effective way in which they can be recognized
'Personally communicated.
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and destroyed on roots used for propagation, even assuming they are spread in this way. The most effective means of control is to eradicate promptly and thoroughly the first indications of disease that appear. The old stalks should be cut off in the fall below the ground-line, not merely pulled, which usually leaves the infested parts still present; and all such debris should be carried out of the garden and burned.
If the plant has been attacked by Botrytis, the soil over and around the crown should be carefully cleared away, and replaced with uncontaminated soil and a top covering of sand or cinders added. At this time some growers prefer to mix hydrated lime with the soil used for replacement. If a winter mulch is used, it should be removed as soon as the shoots start into growth in the spring. Any shoots which show a discolored, decayed area at the base should be cut off below the ground-line and burned. This examination should be periodically repeated, and must be thorough. The first bud- and leaf-infections also should be promptly removed. Ants, which are active in carrying spores from the decaying basal portions to the buds, may be discouraged somewhat by frequent clean cultivation about the crowns. Other Blights and Leaf-Spots.
Another foliage-blight, which resembles that caused by Botrytis, is caused by the fungus Phytophthora. It has been found in several localities and may occur in many more, possibly being mistaken at times for Botrytis. In general, the affected parts are darker brown or nearly black. The buds and foliage seem to be attacked first, but under suitable weather conditions entire shoots may be blighted. Resting spores are produced in the infected parts, which are probably the means of surviving the winter, but whether the same fungus also invades the crown, penetrating and causing rot there, is as yet unknown. Fungi of this type are very susceptible to the toxic action of copper so that spraying with Bordeaux would probably prevent the destructive occurrence of this disease, except in very wet weather. According to Auten, a satisfactory Bordeaux mixture for peonies consists of i pound of copper sulphate and % pound of hydrated lime to 50 gallons of water. It is said to be hardly discernible on the foliage yet is effective in reducing bud-blight. The copper-lime dust recommended for control of Botrytis blight would also be effective against this disease.
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A number of other leaf-spot diseases of peonies have been described. One of these, Cladosporium spot, characterized by the appearance in midsummer of large blotches, purple-brown above and chestnut-brown on the under-leaf surface, is of frequent occurrence though it usually causes but little injury. Another leaf-spot, caused by the fungus Septoria, attacks both stems and leaves, producing small, brown, purple-bordered spots, with ashen-gray centers in which minute black specks appear. In the peony garden at Macdonald College, Quebec, in 1921 and 1922, 20 per cent of the plants of all varieties were affected. It occurs from midsummer onward. Whetzel has described as Anthracnose a disease having similar symptoms but of unknown, though probably fungous origin. Such infection of peony foliage is favored by cloudy, humid, or wet weather, by inadequate spacing, by too rank a growth, and by the presence of the remains of disease-infested plants. The corresponding opposites—dry, clear weather, proper spacing, and clean culture—usually suffice to prevent serious injury, but the use of a protective spray or dust in addition may become necessary if unfavorable conditions have already developed.
Sclerotinia Stem-Rot.
Peonies are occasionally attacked at the ground-line by other fungi which, like Botrytis, form granular resting bodies, or sclerotia, in and on the decayed parts and in the soil. As these sclerotia remain infectious for a year or more, it is important to eradicate badly diseased plants promptly and remove the soil which the diseased root occupied. One form of stem-rot is caused by the fungus Sclerotinia, a common and omnivorous parasite of garden and greenhouse plants. Wet weather and humid conditions are essential to its attack. The stems are rotted off more or less completely at the ground-line, causing sudden wilting of the aerial portion. Within the pith of stems killed by this fungus, rather large, irregular black sclerotia are formed.
The peony may also be attacked by the Sclerotium fungus that causes delphinium root-rot. The symptoms are similar to those just described but the sclerotia are reddish brown in color, usually globular and smooth.
The fungus Verticillium also has been recorded on the peony, causing, as it does in many other plants, among which are
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dahlia, tomato, potato, raspberry, lilac, and maple, a gradual, progressive wilt that finally becomes complete. This fungus may persist in the soil for several years, hence the appropriate remedy is to dig out affected plants, roots included, and destroy them. A new site should be found for replacements, or a generous quantity of soil excavated from the old site before a new plant is introduced. Root-Rot.
Local necrosis and decay of the underground parts are undoubtedly important factors in the unthrifty growth and failure to bloom of many peonies. No comprehensive study has been made of the extent of this sort of injury or of the role of various fungi that are frequently associated with it. Deep planting, heavy mulching, and the use of manure as a fertilizer have not been studied in relation to root-rot, but most growers believe that these practices are conducive to root-injury.
Sometimes not the fleshy roots but the crown or collar is the seat of fungus attack. This may result from unfavorable cultural conditions such as those just mentioned, but is doubtless often the outcome of neglecting to remove diseased and injured shoots before decay spreads from them into the crown.
The fungus Rhizoctonia, a soil organism that attacks the roots of a great variety of plants and causes such well-known diseases as potato stem-rot, brown-patch of turf grasses, damp-ing-off of forest seedlings, and the like, is present very frequently on the roots, crowns, and stems of diseased peonies. On the large, fleshy roots it seems to produce a characteristic rot which usually centers around a branch rootlet. The infected area is roughly circular, depressed, and nearly black. It is usually shallow, extending in only to the woody cylinder, but sometimes large roots are rotted through. This appears to be a well-marked disease though possibly it is initiated by rootlet injury resulting from the over-use of manure. Roots variously deformed, as a result of root-knot or gall, and crowns which are too deeply buried are particularly subject to decay. Shoots arising from such roots are often partly girdled by a dark-colored area of decay at the base, and the flower-buds invariably fail to open. Rhizoctonia has been found associated with this condition many times, but whether as the sole and immediate cause or as a weak parasite following some other injury is unknown.
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Enough has been said of the cultural conditions which favor root-rot so that the appropriate control measures may be applied. In particular, the use of stable manure in contact with peony roots and crowns is to be avoided. Not enough is known of the relation of Rhizoctonia to this disease to justify attempts to control it by disinfecting the soil about the crown. However, if it is desired to save a valuable root which is affected with rot, the decayed parts should be excised, the root soaked in a disinfectant (mercury bichloride, iV per cent solution, or one of the organic mercury preparations, for one-half to one hour) and replanted in a new uncontaminated site.
Root-Knot or Root-Gall.
These terms are often used as synonymous, although root-knot in other plants has come to mean the particular kind of gall caused by the parasitic eelworm, Caconema radicicola. This eelworm, or nematode, is often found in small knots or galls on the minor roots of peonies, particularly during the summer after many new roots have been formed. Again, galls which appear superficially identical are present, yet no nematodes of the root-knot type can be found. The fact that this disease is more prevalent in southern than in northern regions is in accord with the view that the root-knot nematode is the cause, yet the difficulty so often encountered in demonstrating that this parasite is present in affected roots makes it unsafe to conclude that this is the full explanation.
A somewhat different type of root-deformity is that in which the fleshy roots are swollen, stubby, and abruptly constricted at the ends or present a series of small, bead-like swellings. There are usually only a few, short, more or less rotted fibrous roots. The examination of such specimens usually fails to reveal the presence of root-knot nematodes; and it is sometimes assumed that if eelworms are implicated they were present only in the initial stage of the attack while their effects persist, or that this is a different kind of disease.
The gnarly, irregular form of peony crowns which are too deeply buried has been mentioned. Another form of overgrowth of the crown and roots resembles the plant tumors, called crown-galls, which occur in various nursery plants, as fruit trees and shrubs, roses, chrysanthemums, etc. These galls result from infection by a specific bacterial parasite, which gains
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entrance through propagating wounds and slight injuries during cultivation, but not much is known about this type of infection in peonies.
The plant symptoms associated with these various root-deformities are: (i) Absence of bloom, or few and weak flowers; (2) spindly stems and thin, small leaves which are usually rolled lengthwise; (3) pale green color.
The means of prevention and control are: (i) Avoid planting roots which show a galled or knotted condition; (2) if plants develop these top symptoms, investigate the condition of the root, discarding it if badly diseased or dividing it into small (one-eye) portions which should be transplanted to a different site, if the plant is not badly diseased and is considered of sufficient value to warrant the extra care; (3) the soil in which an infected plant grew should be excavated and replaced with fresh soil; (4) if this trouble becomes general in a planting, disappointment and loss of time will be saved by discarding the old plants and starting a new bed elsewhere with healthy stock. Needless to say, growers of peony roots for sale should cooperate to eliminate this source of dissatisfaction by cleaning up their own plantings and withholding all diseased roots from the market.
Experiments are now in progress in the United States Department of Agriculture to determine whether a hot-water treatment, similar to that used to free narcissus bulbs from stem-infesting nematodes, may not be employed to control root-knot of peonies. Another method of destroying the nematodes within the galls is being tested, which involves drying the roots for various lengths of time before resetting them in clean soil. Although promising results have been obtained, this work is still in the experimental stage, and no treatment can as yet be recommended which will insure destruction of the nematodes and avoid injury to the roots.
Miscellaneous.
Several types of injury or disease of minor importance remain to be mentioned. Peonies occasionally exhibit a peculiar sort of mosaic infection. Affected leaves show characteristic yellow or light green spots and rings. The disease seems to be like the mosaic infections of other plants in that once present it persists as long as the plant remains alive. It spreads slowly, however,
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involving at first only a few leaves or shoots and gradually affecting the entire plant, but it does not in the early stages weaken the plant conspicuously. It is not known how it spreads to other plants.
A type of foliage distortion has been observed several times in which the shoots are foreshortened and the leaves are crinkled or curled, giving the general aspect of a dwarf plant with exceptionally dense foliage. Such plants are shy bloomers. There is some evidence of spread from plant to plant, and affected plants are said not to recover even when transplanted. The roots appear normal. These symptoms are suggestive of the systemic leaf-curl diseases known in other plants.
Small insects, called thrips, sometimes infest peony buds and flowers so severely that the buds fail to open and the flowers are ragged, spotted, and short-lived. Badly infested buds might be mistaken for those blighted by Botrytis. These insects multiply in weeds and trash, so that proper spacing and clean culture are often an effective means of combating them. Not much can be done to help plants which are already badly infested but if the first spotted buds are picked off and burned, a general attack can usually be prevented.
A PATHOLOGICAL QUESTIONNAIRE
editor's note.—One of the members of the Society, knowing the Manual was in the process of compilation, wrote to the Editor asking that answers to the following questions be included. These answers were very courteously supplied by Dr. Weiss, author of the foregoing article, with the assistance of Dr. Steiner of the Office of Nematology, also of the United States Department of Agriculture.
Nematodes or Root-Gall
Q.—Is the peony infested by more than one kind of nematode?
A.—The peony is doubtless infested by more than one kind of nematode, but at the present time only one nematode is known that causes serious injury, namely Caconema radicicola, the gall nema.
Q.—Does the nematode which infests the peony also infest other plants? If so, what ones? Iris? Narcissus? Strawberry?
A.—Yes, about 670 different host plants belonging to as many different species are known today to be infested by Caconema radicicola, the gall nema. Among these is iris, as well as strawberry. Narcissus is not known to be a host plant.
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Q.—Does the nematode reproduce both in the soil and the peony roots? If not, in which?
A.—The eggs of Caconema radicicola are produced by the female nema inside the roots of the peony. If the root decays, the eggs become free and hatch in the decaying matter. The larvae enter the soil and seek out new young roots of the peony. Reproduction occurs only inside the roots of the peony or the host plant, so far as is known at present. Males grow up and mature inside the host plant, then seek out and fertilize the females.
Q.—Does the nematode thrive best in sour or alkaline soil, dry or moist?
A.—Neutral soil is preferred by Caconema radicicola, but it thrives also in moderately sour or moderately alkaline soil. A moderate amount of moisture is preferred but the nema thrives in comparatively dry as well as comparatively moist soil. If the dryness is too great, the nema may be killed. A certain not extreme amount of dryness causes the nema to go into a quiescent condition from which it may revive after dry spells of very considerable length. The nema may be drowned by being submerged in water several weeks. Where land is flooded, many of the nemas are drowned if the flooding lasts for several weeks, say at least six weeks, but many of the nemas enclosed in roots and in spaces where there is air survive this drowning treatment.
Q.—How much cold is required to kill nematodes wintering in peony roots? How much heat in hot-water bath? How much exposure to X-rays? Does exposure to X-rays or cold kill the eggs?
A.—Soils annually frozen for a considerable time to a depth of 6 to 10 inches are by this process freed of Caconema radicicola, with, perhaps, rare exceptions. There seems good reason to believe that peonies properly mulched will stand an amount of cold in the winter which would destroy the Caconema radicicola, but decisive experiments in this respect have not been carried out. No experiments have been made with peony roots to ascertain what degree of heat will kill the nemas in the peony roots. There is some evidence that ordinary X-rays do not effect Caconema radicicola, but the experiments were not carried out upon peonies. Cleanly washed, infested roots of live plants were entrusted to the strongest X-rays that were available in Washington laboratories fifteen years ago. None of the treatments seemed to affect any stage of the nema. The plants were not killed and were set out and grew. Apparently they were as thoroughly infested as the check plants, showing that the nemas were not even rendered sterile. Since that time, however, rays, different from those tried in the experiments, have been discovered, explored, and made available. What may be theeffect of these new rays thus made available is not known.
Exposure to the sun's heat in climates like that of much of California, New Mexico, and Arizona kills the nema. A dust-mulch stirred frequently will result in killing off many of the nemas, but they cannot be thus exterminated. In land that has become thoroughly infested in these regions, the nemas have gone as deeply as the roots of the plants
Root-knot of peony caused by parasitic eelworms—the root-knot nemutode
plate XXIII
Type of spindly growth and small, narrow, rolled leaves associated with root-knot. The swollen and stubby large roots and the absence of fibrous roots are evident.
Above—Root-gall (Lemoine disease) (Photo from F. H. B.)
Below—Peony root with early stage ot so-called I.emoine disease (Photo by N. A. Brown, U. S. Dept. of Agriculture)
plate XXIV
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and it is manifestly impossible by any practicable agricultural operation to bring such deep soil to the surface so as to subject it to sunlight. Hence, when new crops are grown in land that has been sunned, the nemas from deeper down migrate toward the surface and infest the new crop. Such treatments, nowever, are beneficial enough under a considerable number of circumstances to make them pay.
Q.—What is the number of broods or life-cycles per year?
A.—Under favorable climatic conditions a brood or life-cycle is a six-weeks' affair, or thereabouts. In the northern ranges of the pest there are probably about three broods per season. In the southern portion of the United States reproduction is continuous, though the life-cycle is somewhat prolonged in the colder portion of the year.
Q.—Do they spread more rapidly where there is a ground-cover, like sod or strawberries?
A.—It depends on the ground-cover. Most grasses are more or less resistant to Caconema radicicola and it does not spread rapidly where there is a sod composed of resistant grasses. Strawberries are not resistant.
Q.—Can they be killed in the soil by the Semesan or Uspulun treatment?
A.—Such treatments have a beneficial effect but do not constitute a cure. So far as known, the beneficial effect is not very great.
Lemoine's Disease.
Q.—What is it? Fungus, some especial kind of nematode, or merely advanced nematode infection?
A.—The nature of this disease is unknown. Some hold that nema-todes have nothing to do with it since at least the recognized parasitic species, and in particular the root-knot eelworm, are typically absent from the root-swellings which characterize this disease. There is also no evidence of fungous infection in typical specimens. It has been suggested that the swellings originally resulted from nematode infection of very young roots, from which the nematodes later migrated, while the effects of their feeding persist, but this has not been substantiated. This disease has nothing to do with crown-gall.
Q.—Does it spread in the soil or only by contact infection, say from knife used in dividing?
A.—It is generally assumed that this disease is introduced by planting infected roots and that it spreads through the soil. There is no evidence that it is transmitted on the knife used in dividing roots.
Q.—Is there any known cure for it?
A.—There are several references in horticultural literature to the possibility of eliminating Lemoine's disease by pruning out the affected portions of a root, reducing the remainder to one-eye cuttings, and setting them in a new site. The details of such experiments are not given, and it is not clear that any control plants were grown,
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Botrytis.
Q.—Are the Botrytis of the peony and the tulip the same? and intercommunicable ?
A.—The species of Botrytis which attack tulips and peonies are different and distinct. It has been shown that the tulip Botrytis does not attack peony foliage.
Q.—Does it cause any decay of underground root or crown tissue?
A.—Whetzel states that Botrytis may extend through the crowns into the fleshy roots, resulting in a wet brown rot, but that sclerotia have not been observed on diseased roots. My observations have been very limited but I have never isolated Botrytis from naturally infected specimens of root-rot though Rhizoctonia or Phytophthora may be readily obtained. Botrytis may extend into subterranean portions of stems, and into the crown, though the sclerotia are typically formed just above the ground.
Q.—Does it winter over on underground root or crown tissue, or only on leaves and stems?
A.—I do not know of any definite evidence that Botrytis lives over winter in the crown or root. Affected crown tissue is pretty well decayed by fall, and in the absence of the formation of sclerotia, it at least remains to be proved that the fungus mycelium lives over in the decayed parts. The sclerotia may readily get into the soil, where upon germination they give rise to spores which can infect young shoots.
Wilt.
Q.—What disease causes peonies which show no root-gall or Le-moine's disease to wilt down early in August (no blight apparent), say in a year or two the stalks get smaller, plant fails to bloom, and just naturally "goes bad" with no disease apparent. Buds may or may not get more numerous (i. e., underground buds), elongated, and possibly more pointed. Foliage may get reddish in midsummer in first stages also. Is there any cure? Edulis Superba will do this quite often.
A.—The symptoms suggest Verticillium wilt, which is described in the article on peony diseases. The only recourse is to dig out all of the affected root and destroy it. The soil should be thoroughly excavated also and replaced by clean soil if another peony is to be reset. A similar disease also attacks maples, lilacs, raspberries, and many garden plants. This should be borne in mind when selecting a peony planting-site.