WILT DISEASES. Acremonium (Cephalosporium)

Transcription

1 WILT DISEASES To wlt means to lose freshness or to become flaccd. Wltng n plants may be temporary, due to too rapd transpraton; or t may be permanent, due to contnued loss of water beyond the recovery pont. Dsease organsms, by reducng or nhbtng water conducton, may cause permanent wltng. Because wlt dseases are systemc, and ted up wth the entre vascular system of a plant, they are usually more mportant, and harder to control, than localzed spots or cankers. In many cases the fungus enters the plant from the sol through wounds or root hars and cannot be controlled by protectve sprayng. Often, although the fungus s present only near the base of a plant, the frst symptom s a flaggng or wltng or yellowng of a branch near the top. Many speces of Fusarum are responsble for mportant wlts and yellows. Vertcllum s a common cause of wlt n maples, other trees, and shrubs, but most mportant among the wlt pathogens are two speces of Ceratocysts, one causng oak wlt, the other Dutch elm dsease. Acremonum (Cephalosporum) Leaf Spots. Acremonum dospyr (formerly Cephalosporum dospyr). Persmmon Wlt, a lethal dsease of common persmmon. Wlt appears n scattered localtes from North Carolna to Florda and west to Oklahoma and Texas, but most nfecton s n north central Florda and central Tennessee. Spread s rapd and death quck. Frst notce of the dsease was n Tennessee n By 1938 only 5% of the persmmons n the nfected stand were alve. Topmost branches wlt suddenly, then the rest of the tree, wth defolaton and death. The fungus fruts n salmon-colored spore masses n cracks n dead bark of dyng trees or under bark of dead rngs. Fne, blacksh streaks are present n fve or sx outer rngs of trunk, branches, and roots. No control s known.

2 682 Plant Dseases and Ther Pathogens Cephalosporum dospyr (see Acremonum dospyr). Persmmon Wlt, a lethal dsease of common persmmon. Cephalosporum sp. Sunflower wlt. Ophostoma (Ceratocysts) Ascomycetes, Ophostomatales Pertheca enlarged at base, wth thn walls, and long slender neck, ascus wall evanescent, ascospores hyalne. Condal stage may be Chalara wth endogeneous spores or Graphum wth external conda or condophores unted nto a dark stalk (synnema). Ceratocysts fagacearum (Chalara quercna, Endocondophora fagacearum). Oak Wlt, our most serous dsease of oaks, now known n 20 states from Texas and Oklahoma east to Pennsylvana and South Carolna. It has also been reported n Florda. Although apparently present n the Upper Msssspp Valley for many years, the dsease dd not cause concern, and the fungus was not descrbed untl 1943, snce when t has become a major threat to our forest economy and to trees n resdental areas. All natve oak speces are susceptble, also chnquapn, chestnut, lthocarpus (and apples n expermental noculaton); but red oaks succumb most rapdly. Scoutng for the dsease has been done largely by arplane, the dscolored folage beng vsble up to a half mle. Frst symptoms are a slght crnklng and palng of leaves, followed by progressve wltng, bronzng, and brownng of leaf blades from margns toward mdrbs and defolaton progressvely downward and nward throughout the tree. Red oaks almost never recover and may be klled wthn 4 to 8 weeks after symptoms appear. Whte and burr oaks may persst for some years, wth affected branches dyng n a staghead effect. The frst nternal symptoms are the formaton of gums and tyloses n the xylem. After wltng, mycelal mats are formed between the bark and wood, and the bark cracks from the pressure exerted. Pertheca are formed n these mats, whch have a sour odor and attract nsects. Ntuld beetles, frut fles, brentds, sprngtals, bark beetles, and possbly other nsects get conda and ascospores on or n ther bodes as they feed, and can noculate other trees through wounds. We know that ascospores reman vable several months on nsects and can be dstrbuted through fecal pellets, but we do not yet know how great a role they play n the spread of oak wlt. Brds have been suspected as carrers but are not yet ndcted. Local spread s largely by root grafts,

3 WILT DISEASES 683 one tree nfectng others wthn 50 feet and wth grafts possble between red and whte oaks, not lmted to the same speces. Control. In resdental areas nfected trees should be removed. In forests, fellng may wound other trees and spread the dsease more than lettng the dead tree reman but treated so that t s not nfectve. Dfferent states handle the problem n dfferent ways. In Pennsylvana, each nfected tree s cut, wth all other oaks wthn 50 feet, and ammate crystals are placed on each stump. In North Carolna stumps and felled trees are thoroughly sprayed. In West Vrgna the trees are left standng, but a deep grdle nto the heartwood dres out the tree so that mycelal mats and spores do not form. Ceratocysts (Ceratostomella) ulm (Graphum ulm) (see Ophostoma ulm and O. novo-ulm). Dutch Elm Dsease, on Amercan, Sbran, Slppery and European elms n 31 states, Mane to North Carolna and west to Oklahoma, and on cedar. Ophostoma ulm and O. novo-ulm (formerly Ceratocysts (Ceratostomella) ulm (Graphum ulm)). Dutch Elm Dsease, on Amercan, Sbran, Slppery and European elms n 31 states, Mane to North Carolna and west to Oklahoma, and on cedar. Ths fatal dsease s not really of Dutch orgn but s so named because t was frst nvestgated n Holland. It was notced n Europe about 1918, frst n France, then n Belgum and Holland. It spread throughout central and southern Europe, then nto England and Wales. In many places t vrtually extermnated the elms, ncludng those on the famous avenues at Versalles. It s suspected that the fungus came to Europe from Asa durng World War I. Dutch elm dsease was dscovered n Oho n 1930 and n New Jersey n It has spread north through New England and has become very serous n the Mdwest. In 1948, the dsease was found n Denver, Colorado, and n 1976 n Calforna. It s now farly wdespread n reports of ts occurrence n the Unted States. The spread of the fungus s lnked wth the presence of the large and small European bark beetles, Scolytus scolytus and S. multstratus. Only the latter s establshed n ths country, havng arrved n Boston about Patent detectve work establshed the fact that the fungus came here n elm burl logs mported for furnture veneer. After one such nfected elm burl was found n Baltmore n 1934, months of scoutng went on n the vcnty of ports of entry, ralroad dstrbutng yards, and veneer plants. Such backtrackng showed the nfected materal had come n at four ports of entry and had been carred by 16 ralroads over 13,000 mles n 21 states. From ths source the dsease got ts start n at least 13 areas n 7 states.

4 684 Plant Dseases and Ther Pathogens Elm nursery stock s, of course, quarantned, and elm burls are embargoed; but who who would have beleved that dshes could have anythng to do wth kllng our elms? Dshes have to be crated, however, and several tmes snce 1933 Englsh dshes crated wth elm wood carryng bark beetles and Ceratocysts have been ntercepted. All Amercan and European elms are susceptble. Asatc elms, Ulmus parvfola and U. pumla, are resstant. A seedlng elm, named Chrstne Busman for ts Netherlands dscoverer, s hghly resstant, though not mmune, and s now avalable. Other promsng seedlngs have been tested by the U.S. Department of Agrculture. Symptoms are apparent from the latter part of May untl late fall. The acute form of the dsease s characterzed by sudden and severe wltng. Frst the young leaves, then all leaves wlt and wther, sometmes so rapdly that they dry, curl, and fall whle stll green, before they can turn the usual brown of dead leaves. Sometmes termnal twgs are curled nto a shepherd s crook. Chronc dsease symptoms are gradual, often takng all summer for complete defolaton. In many cases ndvdual branches or flags appear, the yellowed leaves conspcuous aganst the rest of the tree; but sometmes all leaves gradually turn yellow. In another type of chronc dsease, trees leaf out late n sprng, wth sparse chlorotc folage and a staghead appearance. When an affected twg s cut across, the vessels or water-conductng tubes show dark brown or black, beng clogged wth bladderlke tyloses and brown gummy substances (see Fg. 3.62). The producton of these substances s thought to be stmulated by a toxn secreted by the fungus and carred n the sapstream. Symptoms are not dependent on the physcal presence of fungal hyphae n all parts of the tree. The fungus lves n the sapwood, frutng n cracks between wood and loosened bark and n bark beetle galleres under the bark. Ths frutng s of the anamorph state, spores beng produced n structures called corema. These are black stalks about 1 mm hgh wth enlarged heads bearng vast numbers of mnute, pear-shaped spores embedded n a translucent drop of stcky lqud. Spores n the vessels ncrease n a yeastlke manner. The perthecal stage, not found n nature, has been produced n culture by crossng plus and mnus strans of the fungus. Although the smaller European elm bark beetle s chefly responsble for spread of the pathogen, at tmes the natve elm bark beetle, Hylurgopnus rufpes, s the agent. When the adult beetles emerge from under the bark of dead or dyng trees, they brng along stcky spores on ther bodes and depost them as they feed n the crotches of young twgs or leaf axls of near-

5 WILT DISEASES 685 Fgure 3.62 Dutch Elm Dsease. A branch cut to show dscoloraton of wood; B wound n twg crotch due to beetle feedng; C bark-beetle carrer of the fungus; D egg and larval galleres of the beetle engraved on sapwood by healthy trees. Although the beetles feed on healthy wood, usually wthn 200 feet of ther orgnal tree, they breed only on weakened or dyng wood and may fly some dstance for t. The European female tunnels out a brood gallery 1 or 2 nches long n the wood, and when the larvae hatch, they tunnel at rght angles across the wood (Fg. 3.62). There s a second brood n August and September, but the overwnterng one, emergng n May, s most to be feared. Because the dsease often follows traffc routes, automobles probably account for a good deal of long-dstance spread. So far as we know, the only other natural means of nfecton s by root grafts, made when trees are planted so close together that ther roots touch. Ths s another argument for dversfed plantng, rather than streets closely lned wth but one type of tree. Control. In the frst few frantc years an enormous amount of money (more than $26 mllon) was spent on tryng to eradcate the dsease by removng and burnng dseased trees; and whle ths was undoubtedly helpful, t dd not stop the spread of wlt. The Federal government has now left the control of Dutch elm dsease up to the communtes and s restrctng ts efforts to research. Many towns have taken a lassez-fare atttude, thnkng that our elms are doomed anyway, so why waste money? Other, more enlghtened communtes have proved that a sustaned control program keeps the dsease down to a neglgble 1 or 2%, or less, and that the cost s far, far less than that of contnuous removal of dead trees. The Mdwestern Chapter of the Natonal Shade Tree Conference, n ts Gude for Communty-Wde Control of Dutch Elm Dsease, suggests: 1. Survey of the total elm tree populaton to be protected.

6 686 Plant Dseases and Ther Pathogens 2. Symptom scoutng for detecton of dseased trees and santaton scoutng for badly weakened elms and wood ples contanng elm wood. 3. Destructon of known sources of elm wood actually or potentally hazardous for spread of dsease. Elm wood ples should be destroyed completely, or each log strpped of bark and the bark destroyed. Dseased trees should be burned, on ste f possble, or thoroughly sprayed. Wood chps from dseased elms may stll carry the fungus; materal should be burned, not used for mulches. 4. Sprayng of healthy trees to prevent nfecton. 5. Mantenance of elms n healthy condton to prevent nvasons of bark beetles. Ths ncludes proper waterng and fertlzng, sprayng to control summer folage pests f necessary. A sngle annual DORMANT SPRAY s now consdered suffcent to protect healthy elms from bark beetles f enough materal s used and complete coverage s obtaned. Ths spray was orgnally a very heavy dosage of DDT, whch caused some brd mortalty and other envronmental problems. Some communtes, of whch Greenwch, Connectcut s a good example, fgure that they cannot afford not to spray, for t costs less to spray for control than to remove a dead tree. Where dormant sprayng and santaton have been combned consstently, the annual loss from Dutch elm dsease has been kept to 1% or less. Chemotherapy, njecton of chemcals that wll nactvate the fungus, has been a promsng lne of research for many years. A parastc European wasp s now beng bred at several laboratores for release aganst the bark beetles. To have elms n our future we must keep on plantng them. Some forms, such as the Chrstne Busman and Groeneveld elms, are qute resstant although not mmune. Chnese and Sberan elms are resstant. Dothorella Cankers. Dothorella ulm. Dothorella or Cephalosporum Wlt of elms. Deback, rather common on Amercan elms, occasonal on slppery and Sberan elms n central and eastern states. The names are confusng. In culture the fungus develops spores as n Cephalosporum, but n nature Dothorela-type pycnda are developed on bark of klled twgs. The fungus has also

7 WILT DISEASES 687 been classfed as Deuterophoma. Spores are extruded n a stcky mass and are dssemnated by wnd, ran, possbly nsects. Infecton s through nsect or other wounds on folage. The mycelum proceeds from leaf petoles nto wood, where t s confned to the vessels. The folage wlts and yellows; there are gradual dyng back of the crown and a brownsh dscoloraton n outer rngs of the wood. Wthout laboratory dagnoss the dsease cannot be postvely separated from Dutch elm dsease, but the ellptcal cankers on the stems, wth small black specks of pycnda, provde one dagnostc symptom. Older trees de 3 to several years after frst symptoms; nursery trees, n 1 or 2 years. Some trees recover, and some reman nfected for many years wthout showng much effect. Control. Prune out nfected branches a foot or more below the lowest pont of dscoloraton. Promote vgor by feedng, waterng, aeratng sol. The ncluson of a fungcde n sprays for elm-leaf beetles or cankerworms mght be helpful. Fusarum Rots. Fusarum annuum (F. solan). Fusarum Wlt of chl pepper. Underground stems are dry, brown, but the roots soft and water-soaked; plants wlt and de rapdly. Spores are spread n rrgaton water and wth wnd-blown partcles of sol. Avod heavy, poorly draned sols. Fusarum foetens. Wlt on begona. Fusarum oxysporum. Wlt on pyracantha and basl. Blght and Wlt on purple coneflower (Echnacea). Fusarum oxysporum f. sp. ap. Celery Wlt, Yellows, general n northern celery dstrcts. There are three strans of the fungus, all causng stuntng, vascular dscoloraton, crown and root rot, but one form causes the entre plant to turn yellow at hgh temperatures, producng brttle stalks wth a btter taste. Another stran causes downward curlng of young heart leaves, and the thrd produces no above-ground symptoms except stuntng. The fungus perssts ndefntely n sol. Golden, self-blanchng varetes are more susceptble. Grow green petole celery or somewhat resstant Mchgan Golden, Cornell 19, Tall Golden Plume, Golden Pascal or Emerson Pascal.

8 688 Plant Dseases and Ther Pathogens Fusarum oxysporum f. sp. asparag. Fusarum Wlt of asparagus, a major factor n asparagus declne n Calforna, found n most plantngs. The fungus lves n sol and may be dstrbuted on seed. Fusarum oxysporum f. sp. barbat. Fusarum Wlt of sweet wllam. New growth s yellowed; plants are stunted; leaves pont downward and are tnged wth tan as they de. Roots and lower stem are dscolored brown. Plant n new or sterlzed sol. Fusarum oxysporum f. sp. batatas. Rots. Fusarum oxysporum f. sp. betae. Fusarum Yellows on sugar beet. Fusarum oxysporum f. sp. callsteph. Aster Wlt, one of the most serous dseases of Chna aster, unless resstant seed s used. Plants wlt, wther, and de at any age from seedlngs to full bloom. Older plants are often stunted, wth a one-sded development and a brown dscoloraton of the vascular system. Sometmes all lower leaves are wlted, wth blackenng at base of stem, often wth a pnk spore mass at ground level. Plants n full bloom may suddenly droop ther heads. Such symptoms are n contrast to the mycoplasmalke dsease, aster yellows, where the plant remans uprght, although stunted and yellow. The fungus s seed-borne and perssts n the sol many years. Control. Sterlze sol for seedbeds. Some seedsmen provde seed of wltresstant varetes, but mantanng resstance means contnuous selecton from asters grown on heavly nfested sol under condtons hghly favorable for nfecton, and ths s an expensve process. Fusarum oxysporum f. sp. cattleyae. Wlt of cattleya orchds. The fungus was solated from a prvate collecton n Oho. Leaves wlted, roots abscsed and decayed; flowers fewer, smaller, short-lved. Fusarum oxysporum f. sp. chrysanthem. Fusarum Wlt on chrysanthemums. Fusarum oxysporum f. sp. conglutnans. Cabbage Yellows, Fusarum Wlt, general on cabbage and other crucfers, probably the most destructve dsease of such hosts n the Mdwest, perhaps other sectons. It s serous on cabbage, kohlrab, and collards. Brussels sprouts, caulflower and broccol are moderately susceptble n hot dry seasons. The fungus, whch can lve many years n the sol, enters through the roots, usually rght after transplantng or at the frst hot weather, wth potassum defcency as well as heat thought to favor nfecton. The fungus progresses upward n the xylem, not nvadng other elements untl the plant des. The most strkng symptom s the dull yellow to greensh color of the folage, together wth a warpng or curlng of basal leaves. Leaves are klled and

9 WILT DISEASES 689 shed from the base up; the woody tssue n the stem s brown, wth a watersoaked appearance. The fungus s spread by sol clngng to farm mplements, dranage, water, wnd, anmals and nfected seedlngs. Once the dsease s establshed, general santaton and crop rotaton are of lttle help aganst a fungus that can survve so long wthout a susceptble host. Control. Once sol s nfested resstant varetes offer the only hope. Many have been developed, ncludng Jersey Queen, Maron Market, Wsconsn Golden Acre, Resstant Detrot, resstant strans of Early Jersey Wakefeld, Charleston Wakefeld, Globe, Wsconsn All Season and Wsconsn Hollander. Fusarum oxysporum f. sp. cucumernum. Cucumber Wlt. A newly recognzed form of Fusarum hghly pathogenc to cucumber and muskmelon n Florda, only slghtly pathogenc to watermelon. Fusarum oxysporum f. sp. cyclamns. Fusarum Wlt on cyclamen. Fusarum oxysporum f. sp. cubense. Wlt of banana. Fusarum oxysporum f. sp. danth. Carnaton Fusarum Wlt, Yellows, Branch Rot, general. The frst symptom s a slow wtherng of shoots, often accompaned by change of color from normal deep green to lghter green to pale straw yellow. Plants appear wlted, especally durng the warmer part of the day. Only one sde of the plant may be affected, resultng n dstorton and tendency to curl. If the stem s splt, a brownsh streak s seen n the vascular system. There may be a dry, shreddy rot of affected wood and cortex. Plants may be nfected at any age, but succumb faster f attacked when young. Ths speces of Fusarum does not rot roots; see F. roseum under Rots for the form causng stem and root rot on carnaton. Control. Sterlze greenhouse sol and benches; take cuttngs from healthy mother block; avod overwaterng. Drenchng newly flatted or benched plants has reduced the number of wlted plants but does not replace steamng or otherwse sterlzng sol. Fusarum oxysporum f. sp. erythroxyl. Wlt of Erythroxylum. Fusarum oxysporum f. sp. gladol. Rots. Fusarum oxysporum f. sp. hebae. Fusarum Wlt of Hebe buxfola, and veronca. Reported as kllng nursery plants n Calforna. Fusarum oxysporum f. sp. lactucum. Wlt of lettuce. Fusarum oxysporum f. sp. lycopersc. Fusarum Wlt of tomato, general, n many sectons the most damagng tomato dsease n feld and greenhouse. Chef losses are n states where ar temperatures are rather hgh durng most of the season, susceptble varetes dyng or producng lttle frut. Losses go

10 690 Plant Dseases and Ther Pathogens up to 30,000 tons of cannng tomatoes, or 10 to 35% of the crop n many states. In seedlngs there s downward curvature of the oldest leaves followed by wltng and death. In older plants the dsease s most evdent as frut begns to mature, lower leaves turnng yellow, frst on one sde of the stem or leaflets on one sde of the petole. One shoot may be klled before the rest of the plant shows symptoms. The fungus enters through roots and grows nto the stem, where t produces the toxc substances causng wltng and eventual death. The vascular system n the stem shows a dark brown dscoloraton. In severe nfectons the fungus grows nto frut and seeds, but such fruts usually drop, and seed s not used. Almost all orgnal nfecton comes from the sol, the Fusarum operatng best n lght sandy sols and at temperatures between 80 and 90 F, but the dsease s spread wdely n transplants. It s encouraged by low potassum and hgh ntrogen nutrents. Control. Start seedlngs n clean sol; do not grow n the same land more than once n 4 years. The use of resstant varetes s the chef means of control. Marglobe, Prtchard and Rutgers are moderately resstant, but nfestaton by nematodes may predspose even these to wlt. Pan Amerca, Southland, Homestead and Jefferson are more hghly resstant. Treatng sol wth nematcdes may reduce ncdence of wlt even though the wlt pathogen s not klled. Fusarum oxysporum f. sp. melons. Muskmelon Fusarum Wlt, smlar to that of watermelon, mportant n Mnnesota, New York, New Jersey and Maryland. Seeds rot n sol; seedlngs damp-off; vnes wlt. Fungus perssts n sol and s carred nternally n seed. Varetes Golden Gopher and Iroquos are qute resstant. Fusarum oxysporum f. sp. nveum. Watermelon Wlt, general on watermelon, also on ctron. The fungus s transported n and on seed and perssts n sol 15 to 18 years. It rots seeds or seedlngs, causes wltng of plant, sometmes wth cottony mycelum on surface of dyng vnes. Resstant varetes nclude Improved Kleckley Sweet and Klondke. Fusarum oxysporum f. sp. perncosum. Mmosa Wlt on mmosa from New Jersey and Maryland to Florda. Ths extremely perncous wlt started about 1930 at Tryon, North Carolna, and mmosas have wlted and ded at a rapd rate ever snce. The wlt appeared n one cty block at Morgantown, North Carolna n 1943, and by 1947 trees were dead and dyng on 232 blocks.

11 WILT DISEASES 691 The frst external symptom s a wltng and yellowng of leaves on some of the branches, causng folage to hang down, then de and drop. Death of the tree follows from a month to a year after frst nfecton. The trunk has a brown rng of dscolored sapwood, usually n the current annual rng, and the color may extend out nto the branches. The xylem s plugged wth brown gummy substances. Small branches may have a one-sded wltng wth the bark flattened over collapsng tssue. The dsease has been spreadng n Maryland snce 1947, n Florda snce As wth other Fusara, ths s a sol fungus enterng through the roots, and eradcaton of dseased trees has no effect on spread of the wlt. Nematodes, by ther wounds, may ncrease the ncdence of wlt. Out of a great many seedlngs grown from seed collected from Maryland to Lousana, noculated several tmes wth the fungus and planted n nfested sol, some have remaned mostly dsease-free. These have been propagated by the U.S. Department of Agrculture. Released for commercal sale are Charlotte and Tryon. Fusarum oxysporum f. sp. ps. Pea Wlt, caused by race 1 of ths pathogen and Near Wlt, caused by race 2. Race 1, confned to pea, produces stunted plants, pale yellow green, wth leaves curled downward, stem thckened and brttle near the ground. Plants wlt and de prematurely. The dsease may cause more or less crcular bare spots n the feld, enlargng each year f peas are planted contnuously, encouraged by hgh sol temperature. Some commercal pea varetes are resstant to race 1 but not to race 2. Delwche Commando was the frst varety ntroduced resstant to both races. Fusarum oxysporum f. sp. raphan. Radsh Wlt. Young plants turn yellow and de; others are stunted, wth dscoloraton of roots. Fusarum oxysporum f. sp. spnacae. Fusarum Wlt of spnach. Plants are pale; leaves roll nward, gradually de. The wlt s serous n Texas and Vrgna. One form of the mosac-resstant Savoy spnach s also resstant to wlt. Fusarum oxysporum f. sp. trachephlum. Wlt of cowpea. Fusarum solan f. sp. ps. Wlt of chck-pea. Hendersonula Deuteromycetes, Coelomycetes Pycnda dark, separate; spores dark wth several cells.

12 692 Plant Dseases and Ther Pathogens Hendersonula torulodea. Branch Wlt of walnut. Canker, destructve to Persan walnuts but assocated wth sunburn of affected branches. The fungus s a wound paraste. Phalophora Rots. Phalophora gregata. Wlt of chck-pea. Phomopss Blghts. Phomopss sp. Wlt on ce plant. Phytophthora Blghts. Phytophthora cactorum. Wlt of blue laceflower and baby s breath. Phytophthora cnnamom. Rhododendron Wlt. A wlt of young stock, grafted plants 2 to 3 years old, seldom on older shrubs, most severe on Rhododendron pontcum. The folage s frst dull yellow, then permanently wlted, roots are decayed; stems are brown at sol level and below. Remove nfected stock from frames mmedately; avod excessve rrgaton; keep sol acdty at ph 4.0 to 4.5; provde shade and mulch for young plants. Ths pathogen also causes wlt of Japanese umbrella tree. See under Rots for ths fungus at work on many other plants. Pythum Rots. Pythum myrotylum. Wlt on peanut. Pythum trachephlum. Wlt on lettuce and also leaf blght. Pythum aphandermatum. Wlt of Ncotana.

13 WILT DISEASES 693 Rhzoctona Blghts. Rhzoctona solan. Wlt of watermelon. Sclerotna Blghts. Sclerotna mnor. Wlt of lupne and wld garlc. Vertcllum Deuteromycetes, Coelomycetes Conda one-celled, hyalne, globose to ellpsod, formed at tps of whorled branches and separatng readly from tps. Vertcllum albo-atrum. Vertcllum Wlt, Maple Wlt of many ornamental trees, shrubs, fruts, flowers and vegetables. The fungus was frst solated from potatoes n Germany n 1870 but apparently was present n Calforna as early as It attacks nearly 300 cultvated plants of wdely dverse types and may persst as a saprophyte n the sol 15 years or more. Of the ornamental tree hosts slver maples are most susceptble, then sugar and red maples, elms, wth occasonal reports on alanthus, alfalfa, aspen, ash, boxelder, beech, black locust, camphor-tree, carob, catalpa, Chnaberry, cucumber, deerbrush, dogwood, goldenran, horse-chestnut, Inda hawthorn, redbud, lnden, magnola, oak, osage-orange, olve, pstacho, persmmon, perwnkle, Russan olve, sassafras, strawberry, smoke-tree, tulp-tree, walnut, mango, sunflower and hckory. Maples may wlt suddenly n mdsummer, often a large branch or one sde of the tree dryng and dyng whle the other sde stays fresh. The sapwood of the nfected sde has greensh streaks, and sometmes slme flux develops on the bark. The dsease can be chronc, progressng slowly for several seasons, or acute, affectng the entre tree n a few weeks. In elms the leaves may be smaller than normal, wth a droopng flaccdty n hot hours of the day. Later there s a slght yellowng, deepenng untl the folage s a strkng lemon yellow. Defolaton starts at tme of frst yellowng, and qute often branchlets drop as well as leaves. Sapwood

14 694 Plant Dseases and Ther Pathogens dscoloraton s brown, and the dsease cannot be told postvely from Dutch elm dsease wthout laboratory cultures. Tyloses and gums are formed n the wood as wth other toxn-producng fung. The fungus always progresses upward through the xylem vessels so there s lttle danger of downward nfecton of the man trunk from prunng operatons. Progress s slowed by adequate mosture and by hgh ntrogen fertlzers, ammonum sulfate beng partcularly helpful. Vertcllum wlt s also a problem on rose understock. Ragged Robn, Odorata, and Multflora are very susceptble, Dr. Huey less susceptble, and Manett resstant. In frut trees the wlt s often known as black heart or vertclloss. It s common n aprcots, less so n almonds and peaches; branches may drop ther leaves and de. Also susceptble are sweet and sour cherry, avocado, plum and prune. On bush fruts raspberry, blackberry, dewberry and youngberry the dsease s commonly known as blue stem. The symptoms appear late n the season, leaves turnng pale, cane tps bendng downward, canes takng on a blush color, lower leaves wltng and dryng. Death s often delayed untl the season after frst nfecton. Black raspberres are more susceptble than red. The dsease s sometme serous on strawberres, especally n Calforna, but cannot always be separated from root rots. Plants may collapse n large areas at the begnnng of hot weather. Vertcllum wlt s very destructve to mnt n Mchgan and Indana, also reported, though not so serous n Oregon and Washngton. Infected plants Fgure 3.63 Vertcllum Wlt on Tomato

15 R. Kenneth Horst: Westcott s Plant Dsease Handbook 2008/3/14 18:36 page 695 LE-TEX WILT DISEASES 695 are stunted, defolated, and klled; yeld of ol s greatly reduced. The fungus attacks all speces of mnt, but peppermnt s most susceptble. There are some resstant hybrds. Deep plowng, nvertng the sol, has reduced the amount of wlt. Vertcllum s especally damagng to tomatoes n Utah and Calforna. Frst symptoms are yellowng of older leaves and wltng of tps durng the day; Fgure 3.64 Vertcllum Wlt on Potato

16 R. Kenneth Horst: Westcott s Plant Dsease Handbook 2008/3/14 18:36 page 696 LE-TEX 696 Plant Dseases and Ther Pathogens Fgure 3.65 Vertcllum Wlt on Snapdragon

17 WILT DISEASES 697 later, margns of all leaves curl upward, then leaves drop (see Fg. 3.63). Plants are stunted; frut s small. Moderately resstant varetes Rversde and Essar have been developed for Calforna. Symptoms on potatoes are rather ndefnte, but often there s yellowng of lower leaves, shortenng of nternodes, and rosettng of the top (see Fg. 3.64). Resstant varetes may be symptomless hosts. Vertcllum wlt s common on eggplant and okra, rather rare on pepper. It occurs on Chnese yard-long bean, rhubarb and New Zealand spnach. On herbaceous perennals n eastern gardens I fnd Vertcllum wlt common on aconte and chrysanthemum, wth leaves turnng dark brown and hangng down along the stem. When the stem s cut across near the base, a crcle of black dots ndcates the fungus n the vessels. Such plants seldom de mmedately but flower poorly and gradually peter out. Wlt was serous on greenhouse chrysanthemums untl a wholesale commercal concern started to provde healthy propagatng stock from cultured cuttngs. Other ornamental hosts nclude abutlon, arala, barberry, begona, Chna aster, carnaton, dahla, fremonta, geranum, marguerte, peony, poppy, snapdragon (see Fg. 3.65), stock and vburnum. Control. Sometmes t s possble to prune out an nfected maple and stll save the tree, but often the dyng tree must be taken out. Nether maple nor elm should be replanted n the same spot. Do not transfer plants from areas where wlt has appeared. Do not set raspberres followng potatoes or tomatoes; do not use tomatoes after eggplant or potatoes wthout a long rotaton. Proper fertlzaton and adequate waterng may help trees to recover from wlt. Vertcllum dahlae s consdered by some a synonym of V. albo-atrum and by others as a dstnct speces; reported as causng wlt of dahlas, mnt, margold, ce plant, barley, wheat, oat, potato, Leucospermum, mpatens, gant hyssop, globe artchoke, ash, cabbage, Cnerara and Echnacea, and other plants. Ths form has mcrosclerota and grows on agar at slghtly hgher temperatures. Vertcllum fungcola. Dry Bubble of oyster mushroom. Infecton of sporophores at pn or button stage cause development of typcal dry bubbles; mature sporophores show crackng and curlng of tssues, and depressed, brown, necrotc areas.

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