Volume 13, Part 1, February 1999

Mycology AnswersWhat is the mode of action of fungicides

and how do fungi develop resistance?

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Many factors operate in influencing theestablishment of fungal infections in plants.As a result the occurrence of fungal diseasesis often unpredictable and may be highlyvariable. Crop losses due to fungal activity arehigh both before and after harvesting andmeans of combating fungal pathogens attractsmuch attention. The treatment of plants toprevent such diseases and to eradicatethe causative agent is often by means offungicidal chemicals, although the use ofbiological control systems is now becoming morewidely established. However, there is a greatdeal of controversy about the use of chemicalsowing to concern about effects on theenvironment and other organisms, particularlyin areas where there is intensive agriculturalpractice and large amounts of chemical maybe involved. In addition, importantfungal pathogens (e.g. Phytophthora infestans -potato blight: Botrytis cinerea - grey mould)have developed resistance to some chemicaltreatments.

Some of the older, well established, chemicaltreatments act by protecting the plant(e.g. sulphur, copper, mercury). These chemicalsare applied to the surfaces of plants and act asa barrier preventing spore germination and/or invasion. As a result, the effectiveness ofthe treatment depends on the timing ofthe application to a crop in relation to theoccurrence and spread of the fungus and toconducive environmental conditions. Suchtreatments are not very effective against rootinvading fungi. Inorganic fungicides, such ascopper and sulphur, constitute some of the oldestremedies but still provide extremely successfulcontrol and are highly effective. Bordeauxmixture, a combination of copper sulphateand calcium hydroxide (lime), was used in the1880's to protect vineyards in France againstpowdery mildew disease. Copper-basedtreatments are still important as fungicides(timber treatments) but are also highly toxic toplants and must be used with care. Thesetreatments prevent spore germination by

denaturing proteins and disrupting enzymesynthesis in the fungus. Elemental sulphuris also a very effective chemical treatment,particularly against powdery mildews and rusts,since it interferes with electron transport butit must be used with care because it is alsotoxic to plants, causing loss of chlorophylland disrupting photosynthesis. Other protectanttreatments include the dithiocarbamates(Thiram, Zineb, Maneb) which disrupt aminoacids preventing protein and enzyme productionin fungi and leading to their demise. Thedithiocarbamates are often used as seedtreatments against rust and vascular wiltfungi, but are also toxic to mammals. Ingeneral, these compounds are multisiteinhibitors, affecting a range of enzymes involvedin key pathways which makes them so effective.However, in addition to problems with toxicity,another major drawback to the use ofthese treatments is that any new growth(shoots, new leaves) of a plant will not beprotected as it is formed and also, over a periodof time, the chemicals will gradually be washedoff the plant. As a result re-application, atintervals, may be required for continuedprotection.

Some fungicides can be taken up into theplant, transported throughout the plant tissuesand interfere with fungal growth from within.These are known as systemic fungicides. Intheory this is a good way to ensure thetreatment of the whole plant. Repeatedapplications are not needed to protect newgrowth because the compounds are mobile in theplant. Systemic chemicals are now widely usedas seed treatments and can protect youngseedlings during the most vulnerable part oftheir development. However, there is still aproblem with root pathogens, in most cases,because these fungicides tend to accumulate inshoots, except for the phosphonates that aremore mobile in the plant and provide protectionfrom Phytophthora root rot. Most systemictreatments have a site-specific mode of action, ingeneral affecting a particular biochemical process

or a single step in enzyme synthesis. The firstsy stemic fungicides were antibiotics such asstreptomycin but many are now availablecommercially. Of the benzamidazole fungicides ,benomyl is effectivel y used to combat theactivities of Fusarium spp and powdery mildewfungi. The action is to interfer e with nucleardi vision that eventually prohibits furtheroutgrowth. Some of the systemic compounds e.g.azoles and morpholines affect particular, bu tdifferent, steps in the biosynthesis of sterolswhich are important co m ponen t s of fungalmembranes, The more modern strobilurinfungicides block electron transport III

mitochondria.Site-specific compounds are useful because of

this mode of action and are not as likely to betoxic to the plant. However, because of the sitespe cificity, even a small biochemical change int he fungus may lead to t he development ofre si stance . Resistant strains arise by theselection of resistant individuals or as the resultof sin gle mutations. A resistant st rain may beabl e t o bypass a bl ock in m etabolism andcircumve nt the action of the fungi cide or mayprodu ce unusually large amounts of a specificenzyme to compensate. Fungal pathogens oftenreproduce quickly giving rise to large numbers ofspores in a short time and, as a result, resistancemay develop and spread surprisingly fast ,particularly in instances where the use of the

Volume 13, Part 1, February 1999

chemical has been repeated . Such intensive useof a particular fungicide favours the developmentof resistance by placing a high selection pressureon the pathogen. Important plant pathogenicfungi, that are difficult to control, have developedresistance to systemic fungicides (e .g. Botryt iscinerea - grapes, Erysiph e graminis - cereals ,Bremia lactucae - lettuce, Phy tophtora infestans -potato). There is therefore, a good case for onlyapplying fungicides when and where t hey arenecessary.

Resistance to multisite fungicides is less likelyto develop because a number of changes in thefungus would be required at one time for it tosurvive the treatments.

Fungicides will continue to be important asmeans of disease cont rol bu t it is clea r that adegre e of cau t io n in t h eir appl ica t ion isadvisable. The use of combinat ions of fungicides,or al t ernations of treatments wit h differentmodes of action including multisite fungicid es,a re us eful strategies for minimising thede velopment of resistance. Planting di sea seresistant cultivars and good crop husbandry alsohave a very important role in ge ne ra l cro pmanagement .

Susan IsaacScho ol of Biological Sciences,

Life Sciences Building, University of LiverpoolCrown Street, Liverpool L69 7ZB

British Mycological Society Slide Collection

The So ciety's collection of photographictransparencies of fungi is one of the finest inexistence. The present Slide Committee, currentlycomprising A. W. Brand, A. Henrici, A. R. Outenand D. N. Pegler, meets at irregular intervals andcarefully selects the slides for final inclusion inthe collection. Probably only one in six is eventuallyincorporated. Nevertheless, it is an actively growingcollection, starting with about 50 slides in 1963and rising to several thousand at the presenttime. Following the sad loss of Margaret Holden,who served so efficiently as the Society's slid ecura to r for many yea rs, t he co llection wastransferred to the Royal Botanic Gardens, Kew.It is now housed in the Mycology Building atKew, alongside the Society's library and archives.

The collection exists in order to make slides

available to the membership for the purposes oflecturing and generally promoting mycology. It isnot currently the policy of the Society to releas etransparencies for publica tion, and individualphotographers should be approached. The newmanager of the collection is Gill Butterfill andan y requests for the loan of slides should bemad e t o : M s G. B . Butterfill , c/o the BMSLibrary, The Herbarium, Royal Botanic Gard ens,Kew, Richmond, Surrey TW9 3AE. The loan isfree but the cost of postage needs to be covered.

Examples from the collection are reproducedon the front and back covers of this issue and itis proposed that further samples will appear infuture issues of the Mycologist. The full list isavailable on the BMS Homepage.

D. N. Pegler

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