Fusarium Wilt of Bananas in Jamaica: II. Some Apsects of Host-parasite Relationships

The incidence of Fusarium wilt in a given variety of bananais determined by numerous interacting factors such as geneticallydetermined resistance, physical and chemical soil conditions,and abundance of Fusarium oxysporum f. cubense. To attain markedresistance to wilt, ‘Gros Michel’ apparently requiresa combination of conditions; thus the soil must be alkaline,of medium to heavy texture, and rich in available potash. Nitrogenfertilizing tends to increase susceptibility to wilt. Some striking effects of weather on wilt incidence are describedand various aspects of crop management are discussed: soil deteriorationunder bananas undoubtedly contributed to development of wiltin the past. In the earliest stages of rootlet infection, the pathogen growsmore slowly in the resistant ‘Cavendish’ than inthe susceptible ‘Gros Michel’, probably owing tomore effective tissue reactions in the former. In the field,infections progressing along the root are often checked at theroot base: with ‘Gros Michel’, however, a smallproportion enter the rhizome and as a result wilt generallydevelops. Ability to check such infections, leading to recovery,is only exhibited on very fertile soils. The possible mode of action of various factors on wilt incidenceis discussed, and the importance of vigorous root growth inpromoting host resistance is emphasized.     

Infection Foci of Armillaria mellea in First-rotation Hardwoods

Infection foci of Armillaria mellea, often 10–20 m indiameter, were discovered in first-rotation plantings of oakand beech on former arable or heathland sites—they werecommonly associated with thinning stumps containing the fungus.Since the foci were scattered and contained different mycelialtypes of A. mellea it seemed probable that they had arisen asa result of stump infection by basidiospores. There was evidencethat many years had elapsed between stump infection and theproduction of rhizomorphs, and that subsequent growth of thesedepended upon the presence of suitable living roots. Largeroaks seemed little affected whereas many small and medium-sizedtrees had been killed. Armillaria mellea, infection foci in plantations, tree stumps, basidiospores     

Fusarium Wilt of Bananas in Jamaica: III. Attempted Control

Experiments were set up in an attempt to control wilt in ‘GrosMichel’. Passive measures of eradication and quarantineseldom achieved more than temporary results. Soil disinfectionby fumigants was unpromising. Flood-fallowing, important inCentral America, was not practicable in the West Indies. Applicationto the soil of lime, phosphate, and potash, separately or incombination, had little or no effect on wilt incidence; applicationof nitrogen often resulted in an earlier appearance of wilt.Supplements of various organic materials such as green manures,compost, and bagasse also proved ineffective. Possible reasonsfor these failures are discussed, and various suggestions aremade for the prevention or limitation of wilt in resistant varieties.     

Observations on the Biology of Fomes annosus, with Particular Reference to East Anglian Pine Plantations: II. Spore Production, Stump Infection, and Saprophytic Activity in Stumps

F. annosus produces conidia abundantly in culture but rarelyunder natural conditions. Sporophorea are formed on stumps anddying trees throughout the year provided that the humidity issufficiently high. Spore discharge in the plantations may occurat any time of year, but slowa down or ceases during very dryweather. It takes place at low temperatures, ceasing only whensporophores freeze. In the laboratory, F. annosus basidiospores germinate in relativehumidity exceeding 92 per cent. on freshly cut, unsterilizedpine wood. Viability is lost rapidly in light at temperaturesabove 15°C. Under humid conditions F. annosus spores will infect freshlycut pine stumps. There is evidence that spores of F. annosus washed down intosoil remain viable for at least a short period and that stumpscovered with soil are infected from this source. F. annosusspores can infect stumps for only a few weeks after felling,colonization after a longer interval probably beingprevdntedby competing fungi. In stumps inoculated with a mixed suspensionof F. annosus and Peniophora gigantea spores, Fomes at firstcolonized the wood but was soon replaced by P. gigantea. Otheraspects of competition are discussed. The incidence of natural infection in stumps and the factorsaffecting it are discussed Growth rates of F. annosus on malt agar and in lengths of pineroot are given. The fungus grows in stump roots at about 1 m.per year, and so closely approaches adjacent trees within ayear of thinning the plantation: In large stumps, F. annosusmay survive 15 or even 30 years after felling. Data are givenwhich suggest that the infective capacity of stumps containingF. annosus is greater in alkaline than in acid mils, while replacementby other fungi is slower. Competition of F. annosus with other fungi during colonizationof stumps is discussed. The parasite can grow along roots alreadyoccupied by certain fungi but not along roots containing others.The probable course of succession in stumps rotted by F. annousis described.     

Observations on the Biology of Fomes annosus, with Particular Reference to East Anglian Pine Plantations: III. Natural and Experimental Infection of Pines, and Some Factors affecting Severity of the Disease

Apparently Fomes annosus normally invades healthy, not damaged,roots, the wood being infected almost at once in mall roots,though in larger ones the fungus grows in the bark for sometime. Alkaline soils also favour the latter type of development.Infections of living roots are more frequent around infectedstumps in alkaline soils than in acid ones. Details are given of infection of pine roots induced experimentallyand of the factors affecting invasion. Greater resistance to killing is evinced by older tree and thoseon the margin of the plantation. Disease is leas severe in EastAnglia on acid soils and where there is more organic matteron the soil surface. These relations are apparently associatedwith the types of soil microflora (of which details are given),the antagonism of Trichoderme viride being especially important.     

STUMP PROTECTION AGAINST FOMES ANNOSUS: TREATMENT WITH SUBSTANCES OTHER THAN CREOSOTE

Various substances when added to the stump surface in concentrated solution directly after felling markedly affect the course of fungal colonization. The extent to which applications kill stump tissues probably affects the rate at which saprophytes develop.
Nitrogenous applications favour growth of Trichoderma viride and Penicillium spp., which tend to suppress surface colonization by Fomes annosus ; ammonium sulphamate is especially effective in this respect.
Some other applications are initially toxic to stump fungi but rapid leaching from the stump surface soon permits growth of various moulds, which again tend to suppress F. annosus and to a large extent other primary basidiomycete colonizers. Substances of this type include urea, which encourages growth of Ophionectria cylindrospora , and disodium octaborate, which favours Botrytis cinerea. The probable mode of action of these substances is discussed.
Stumps usually die more rapidly after such treatments than after creosoting, and often as fast as untreated ones. The rapid colonization of treated stumps by saprophytes is of advantage where application is incomplete or the stump is subsequently damaged. The effectiveness of treatments in preventing a major spread of Fomes annosus from roots already containing the parasite is variable and apparently depends to some extent on the particular species becoming dominant.
Preliminary screening may be performed in the laboratory with sections of pine stem inoculated with stump fungi; potential protectants are then tested in the forest.     

Fusarium Wilt of Bananas in Jamaica: I. Some Observations on the Epidemiology of the Disease

The banana wilt pathogen Fusarium oxysporum f. cubense can bedetected in soil by a suitable host test. It often enters thehost through living rootlets, from which it passes into thevascular strand of the main root and thence into the rhizome;apparently infection does not occur through dead roots. The spread of wilt through plantations was studied by takingrecords at 2-monthly intervals: new cases arise both spontaneouslyand in association with pre-existing ones. Flooding is probablyimportant in local dispersal of the pathogen, as it is in long-rangedispersal. The relative importance of some other modes of dispersalis discussed. The soil population of F. oxysporum f. cubense increases considerablywhen wilted bananas collapse and declines shortly after theirremoval. If the site is replanted with a banana variety resistantto wilt the pathogen can thereafter often be detected in thesoil; in the absence of bananas, however, it cannot be detectedby any test after about 10 years, although its continued survivalis well established by many field observations on the incidenceof banana wilt. Little is known about its mode of survival insoil.     

STUMP PROTECTION AGAINST FOMES ANNOSUS: TREATMENT WITH CREOSOTE

Creosoting of pine stumps immediately after felling commonly prevents the entry of Fomes annosus and other primary basidiomycete colonizers through the cut surface. Ceratocystis spp. and Ophionectria cylindrospora frequently enter creosoted stumps, but growth tends to be slow.
Light applications of creosote often fail to exclude basidiomycetes. Failure to achieve protection may also result from delay in application, especially when resin flow from the cut surface is rapid; this flow impedes creosote penetration.
In one experiment creosoting also controlled air-borne Fomes annosus infection of larch stumps. Various protectants were tested in the forest, a preliminary indication of their efficiency being obtainable after 2 months. A gasworks creosote ( b.p. 190–300° C.) was promising, whilst a tar/creosote mixture was unsatisfactory.
Disadvantages of creosote as a stump protectant arise mainly from its retarding effect on stump colonization. Creosoted stumps tend to remain alive longer than untreated ones and the risk of F. annosus attaining dominance in stumps already having root infection is correspondingly greater.