OIDIUM HEVEAE

REPORE 193TH 7 N OUTBREATO K OP HEVEA LEAF MILDEW BY P. BEEI,EY

The general failure of the North-East Monsoon winds to bring rain during the late months of 1936, and the failure of the same monsoon to produce the strong, dry, east winds usually experienced during January-February and the first half of March in 1937, were e mosth t noticeable feature e Malayath f o s n weather beford an e durin e " winterinth g g " seaso f 1937o n . Onl a moderately y dr y spell was experienced during the wintering and refoliation period in 1937. The season November 1936—March 1937, was therefore noteworthy in that it gave an idea, for the first time during ten years of the writer's observations of the Malayan climate, of the typ f weatheo e r Malaya would experienc e even a failurth f n ei o t e monsoonse oth f . Such failur othen i e r worle partth f s oftedso i n followed by extremely drastic results, such as complete failure of growte cropth f , ho failur o ripe t eharvesd an n y e cropb th tr o , destruction of the crop owing to phenomenally rapid spread of diseases or pests. In Malaya, however, failure of the monsoons does not have this effect: the weather in the period in question in 1936/7 was in fact continuously that of the Maritime Tropical Rain Belt, unpunctuated by monsoonal influences. The usual heavy rains of November and December failed to arrive and these months showed rainfalls similar to other months of the year. On the other hand failure of the monsoon winds resulted in the absence of the usual dry period of January and February, instead of which these months showed heavy rainfalls as compared with the same months of previous years. After a moderate rainfall in December, January also e proveonlb yo t dmoderatel t throughouwe y t the country, though certain small areas such as North , Kuala Kubu district and the -Muar coastal regions, proved ex- ceptions and had a dry month, the rainfall being below 5 inches. In February and March the absence of the dry North-East Monsoon winds caused local conditions to have a dominating influence on local precipitation, with the result that freak rainfalls were recorded in many parts of the country. Very high rainfall would be recorded at certain places while at other stations within milew fe sa verrainfallw lo y s were recorded. Similarly freak maximum and minimum temperatures were recorded. Local

140 141

atmospheric, electrical disturbances typical of the maritime tropical rain belt appeared to be chiefly responsible for these freak records. Realisin influence gth winterine th f rai eo n no f rubbego r trees, it can readily be understood that considerable variation in severity of wintering at any one time was bound to occur, as was the case particularly in districts near the foot of the hills west of the main range. The weather of the tropical rain belt,—bright sun with heavy thunderstorms later in the day—appears to favour rubber trees, wite fortunatth h e result tha n 1937i t , despit e absencth e f o e monsoon influences, the trees wintered and acquired a new foliage whic s excellenwa h n coloui t d sizean r , having suffered from mildew onlmila o yt d extent thad an ,t onl certain yi n late-wintering districts which are mentioned in the district reports below. With the assistance of Mr. R. G. H. Wilshaw, Soils Chemist to the Department of Agriculture, S.S. & F.M.S., who also acted as pilot, an aerial survey of the rubber lands west of the main rang s madwa e e earl n Marchyi , wit a hvie determinino wt e gth characte d severit e winterinan rth e f variouo yth n gi s districts, and to ascertain the likelihood of a large area of rubber being in a state of refoliation such that, should weather conditions become suitable, it would be seriously attacked by Oidium Leaf Mildew. This surve s interestinywa than gi t such variation wintern i s - ing and early refoliation as were observed could be attributed almost entirel locao yt l condition weatherf so , soiltopographyd san , of whic soile hth r geologica so l factor greatese th s f seemeo e tb o dt importance e variationth ; n winterini s g followed closele th y general outlines of the geological map shown opposite p. 33 of this Journal, Vol , 19367 . , wit e addeth h d difference that arean i s the latitude of 5° N. were some three to four weeks in advance of thos n i latitud. epassinN ° 2 eg through southern, central Johore. Durin e late-middlgth e portio f Marcno e weatheth h somn i r e districts betwee e foot-hill th coase d th n an t s became rather dull, a suddetherd s an ewa n influ f reporto x f occurrenceo s f Oidiuo s m on the young leaves. The coast was apparently well protected by brigh weathey dr t districtd an r s further inlan brighy db t weather with short intermittent local thunder-storms e lasTh t . weef o k March and the first two weeks of April were generally abnormally duld mosn an i l t t we districts betwee maie th coase nnd th rangean t , resulting in a rapid increase of leaf mildew, though not in very intensive form, being confined usually to the lower shaded branches. A fortunate return to brighter weather in the later half of the month completely stopped any further development of the disease. 142 UUKB&P OF B£POSTS RECEIVED PEG WEEK

35

30

25

20

lO

8 W 8 2 271 1 21 28 18 25 23 FEE. MAR APS East of the range the weather appears to have been rather dry on the whole, though later in the season a dull showery period gave rise to considerable fear that Oidium leaf mildew might assume serious proportions. However, such was not the case, as the usual sunny intervals proved sufficien younw o enablt ne t ge th elea o t f mature rapidly and to throw off the disease. 143

The intermittent bright and dull periods proved favourable to the development of insect pests such as thrips, spiders, mites, caterpillars and leaf-eating pests generally, so that many specimens received for diagnosis were found to be damaged by these pests and associated leaf spot fungi and not by Oidium Heveae. Referenc e tabld grapth an eo t eh show e tim th f sfirst o e - noticed mildew-attack in the various States. The number of reports received during the season, when compared with the previous year, shows that the outbreak occurred ab approximately the same tim n eaci e h year thad e brighan th ,t t interlude th n i e second half of March corresponded with a reduction in the number of report f diseaseso . On March 25th and 26th the writer made a tour by car to inspect the rubber districts north of and in the following week a similar tour of the southern districts was made, in orde o ascertait r e exten th nd severit e leaan tth f f o diseasy e and quality of the wintering and refoliation in the various districts. These observations, together with reports kindly sent in by estate managers, e fordistric e basith mth f o s t reports given below.

TABLE I OIDIUM HEVEAE 1937 REPORTS RECEIVE R WEKDPE K FROM EACH STAT R SETTLEMENO E T

February March April

•^ ^ 1 --i oo : i>~ ^h m 10

Johore 2 4 2 8 6 2 24 Malacca 2 3 7 4 5 1 22 N. Sembilan . . . 7 15 13 4 6 1 2 48 2 3 2 2 14 4 12 39 2 2 8 2 2 16 P. Wellesley ... 1 1 5 7

Kedah 1 1 1 3 1 5 6

Total 2 14 26 25 13 42 25 18 165 144

JOHORE e north-westerTh n coastal district f Johoro s e experiencea d fairly early winterin a resul a rathes f a gy o Decembet dr r d an r January. On March 5th, when examined by air, refoliation was well-advance dvalleye savth pead n ei san t flats where later winter- ing areas were observed. Similarly, central Johore, Kluang to Rengam s onlwa ,y slightly later thae drieth n r coastal districts. North Johore, though rather late, experienced an even and severe wintering of rubber trees. Younger areas were in most cases late d normallan y produce d fallenleaw ha fdne d befor.ol e th e Early in April some excellently refoliated areas free of disease were observed in the northern coastal districts around Tangkah, Muar, Batu Paha Ayed an t r Itam, though occasional patchef o s late-wintering trees in low-lying land were observed to be infected mildey b thriplowee d wth an n i sr branches e rubbeTh . centran i r l and southern Johore carried an excellent canopy of new foliage, late-winterinw onlfe ya g e oldetreeth n ri s rubber being infected with the fungus. Thrips and mites were seen to be attacking some e youngeoth f r clearings. Variatio n qualiti n d siz f leaan yo e f owin o varyint g g fertilit f soilso s yusuall wa , e th y e founb o t d cause of variation in foliage of different fields. In the northern districts, L,abis to Gemas, the wintering was later and less severe, o thas t refoliatio vera f o y s latd mixewa nan e d character, some areas being bar f leafeo , other autumn i s n tintotherd an s s bearing excellent new foliage. Trees in very young leaf were infected by Oidium only in lower branches.

MALACCA TERRITORY

Following a very dry wintering season in the north, the fall beelead d ool ha ff n earl completed yan , being followe rapia y db d refoliation so that the new leaf-canopy was practically complete by the first week in April and few signs of leaf-mildew "were observed, such being slight and present only in lower, shaded branches. The district, however, was later and though much of foliagw s ne ver e wa ey th good som d treeod e s e caughth y b t weathe beed ha rn completely defoliate Oidiuy b d sommn i e cases and by thrips in others. A few trees still carried old brown leaf in April. Younger areas wer vern ei y good conditio t botnbu h Oidium and thrips were foliage w observene presene e b . th o dt Thin o t s was als oe souther th tru f o e ne Territorypartth f o s . Further inland, however, the new foliage was in excellent condition with e writer' diseasy e timth littl an th f o f et i e a es visit. 145

e improvemenTh foliagen i t noticeablo s , 1936n i e , appearo t s have been maintained during the 1937 season, though a slight increase in the number of reports of disease is recorded.

Again winterin d refoliatioan g n followe a coursd e similao t r that of the past few years. Wintering was very severe over most parts while the inland districts at the time of the aerial survey showed a rather poor, pale yellow, new foliage developing in the areas of older rubber. At the time of the second visit this foliage had improved both in size and colour, suggesting that the earlier palenes e foliag th s causef o swa e y lac b df wate o k d mineraan r l supplies to the tops of the tall, old trees. Known manured areas were conspicuou theiy sb r later winter- ing, and in many cases they had new leaf developing before the old d changeleaha f d colou d fallenan r . e pead coastath Treean t n o sl clays were lated moran r e desultor n winterini y d refoliating an ge fina th lt resulbu s , wa t very good, damage caused by Oidium, thrips, mites and spiders being restricted to the lower branches only, and only a few trees were observed completely robbed of their new leaf. e higheOth n r laterite soils nort d sout an hf Por o h t Dickson the trees had wintered early and obtained most of their new leaf befor e e advenratheth th e f o rt more showery weather latn i e March. The later-wintering trees had, however, been attacked by mildew thao s , t lower branches leadevoiw fne werf do e frequently observed. Both Oidiu thripd man s were observe activn di e state in this district. In inland districts, Tampin to Bahau, wintering had been only moderately severe though early, so that some areas had obtaine a dhealth foliagw ne y e while other n valleyi st a r o s the foot of hills seemed to be still in a mixed stage having old and new leaf, brown foliag bar d same an eth e etreen i areal sal . e monoclonTh e block rubbef so r were interestin illustratinn gi g the different inherent characters of the various clones in regard to wintering e chess-boarTh . d effeca larg f o et are f monoclono a e planted rubber viewe s verwa y r dstriking ai fro e square mth On . e woul completele db y bare with white soil showing dazzling though the empty branches, the next square a perfect even russet-brown foliaged ol owine th , anotheo gt trua r e copper owine e th yhu o gt buds all bursting together and exposing the young copper-red leaf, an dnearba y squar l freshal e , pale green, having wintere- re d dan foliated early. Little disease, however, seemed to be present except in lower branches of the latest refoliating trees, and by mid-April the foliage throughout this ares reallawa y good. 146

SjXANGOR During1 1937 the central part of Selangor half-way between e hills,—Kual th e foo f th o t a Lumpu o Kualt r a Kubu,—ane th d coast,—Klan Kualo gt a Selangor, mose th s t —seriouslwa y affected district of all parts of Malaya visited by the writer. The coastal districts favoure longa y b d, bright y speld winteredr , ha l d com- paratively evenly and early, and the foothill regions though later had enjoyed sunny weather with afternoon thunderstorms also favourabl o healtht e y developmen f foliageo t . In the more central districts dull showery weather had delayed winterin e samrefoliatiod th gan t ea timd nan e favoure spreae th d d and virulence of the mildew fungus. Thrips also appeared to be abundant upon the leaf throughout these areas. More north around Kuala Kub n abnormaa u y speldr n li l December and January had induced a very early wintering and early, healthy refoliation o tha s Apriy , b e tdistric th l t stoot ou d s beina advancn gi f otheeo r districts within rang f seriaeo l vision. Generally the great increase in reports of leaf-fall from Selan- gor during 1937 may be said to be caused by the unfortunate climatic experience districte th f so s lying betwee coastae nth l areas and the hills. Late April and May, being abnormally fine and dry, enabled these area mako t s quicea satisfactord kan y recovery. PERAK e districtTh s between Tanjong Mali - d Sungkamex an d ha i perience dlate a mor d an r e desultory wintering with corresponding slow refoliation. Trees lat n refoliatini e g were observee b o t d disease middln di lowed ean r branches. Other variations causey db soil and age were noted, some areas particularly around Slim had obtaine n excellena d t d mucleacanopw an ne f hf o yles s disease was present, though some areas of good new healthy foliage were seen also in the Tapah district. Further north in Kinta a greater variation in stages of wintering and refoliation was observed and is thought to have been caused by nutritional factors. Disease s presen l late-refoliatinwa al n i t g trees examined. North of Ipoh was similarly very variable in condition. Around Sungei Siput, areas varied greatly with soil conditions; fertile land early-refoliatind ha g trees puttin secona t g ou shoodw flusne tf ho and leaf by the first week in April, which in many cases was attacke Oidiumy db t regarde Heifeae,no f t serious thio bu s wa d a s consequence as the canopy of healthy leaf already obtained was considered sufficien neede e treee foliagth th Th f r . s o f small fo eo t - holding rubber trees near Kuala Kangsar was on the whole in good condition, with late trees carryin gmila d attack e districTh . f o t 147

Gunong Semangol was generally poor in foliage owing to poor soil- conditions, late trees being badly attacke resula s da f slo o t w rate of developmen maturityo t t peae Th t. districts were well advanced and thoug hfea w trees near open clearing root-disease-patched san s were diseased the areas generally had good healthy new foliage. coastae Oth f l districts Tron previoue th g n hadi s a s, year rathea , r mixed wintering and refoliation with corresponding, sporadic, heavy attack f mildewso . Bruas showed considerable improvement and thoug t appeahno latd edi r badly diseased. Sitiawa d Soutan n h Perak were also very lat winteringn i e ; though patche d refoliateha s d successfully e majoritd th , ol f o y rubber areas were suffering fro ma mil d attac f diseaso k e which was banished by the sunny weather of April and May.

PROVINCE WEW/ESUSY Rubber tree goon o s d soil thin i s s distric completeld ha t - yre f Marcho d en , foliate e acquirinth y db ga good , heavy, dark green foliage; occasional branches near clearings had suffered defoliation by either thrip r Oidiumo s bothy b r o ., Some areas with poor foliage were observe e latb n i eo t d refoliating and had suffered slight leaf-fall. The wintering generally in the north of Malaya had been less early than is usually the case comparison i , n wit e southerhth n States. KSDAH Though late n winterini r g tha n 1936i n e rubbe,th r treef o s Kedah were in other respects in a similar condition to that reported for that year. The southern parts showed trees refoliating late in which spiders e Bird, e Spomiteth d Ey s t an sfungu s Helminthos- porium Heveae were active especiall youngen yi r areas. n centraI l Keda e somewhath h t late-refoliating trees were attacked by both Oidium and thrips but not to any serious extent. Nort Sungef ho i Patan earle th i y foliag completeld eha y escaped rula e secon diseass eth a d d ean flus f leaf-growto h s attackehwa d by leaf spot fungi and mites and spiders. PAHANG From reports from estate managers in Pahang it would seem thae generath t l improvemen foliagn i t e e previounoteth n di s year had been maintained during 1937. A rather dry March favoured the earlier refoliation and it was not until the later showery wea- ther arrived that reports of the presence of Oidium Heveae were received and it was stated that the lower branches of only the late- wintering trees were affected. 148 Summary During the year 1937 the general improvement in foliage of rubber trees mentione n lasi d t year's repor s beeha t n maintained in most districts, the central parts of Selangor being the most notable exception. e generaTh l e Nortfailurth f ho e East Monsoo o brint n s git usual heavy rains at the end of 1936 and dry weather at the be- ginnin f 193g o gives goo a 7ha typ e s nth u f dwinterin eo o idet s a g and refoliation likely to occur under the normal climatic conditions of the Tropical Rain Forest Belt, the general indications being that such condition e favourablar s e developmenth o t ee rubbeth f o tr tree foliage and unfavourable to an epidemic activity of the leaf- mildew fungus Oldium Heveae. e numbeTh f reporto r s (165e diseasth f )o e received from estates in 1937 is about the same as for the previous year (161). Similarly, owin o continuagt l lac f virulenco ke mildewth f o e - fungus under normal Malayan conditions e sulphur-dustinth , g programmes n carrievarioui t ou d s e countrpartth f o s y were severely curtailed to one or two rounds of dusting, or omitted altogether. The use of sulphur-dusting is finding great favour in the control of thrips, mites, spiders and leaf spot damage in young rubber in valuable rubber-seed-producing areas, and the time is coming when most estates will conside a motor r knapsacko r - duster as an essential part of estate equipment, which will give an added security agains e possibilitth t f heavo y y o t losse e du s Oidium Heveae or other leaf parasites. e Adviseth o t Agriculturen o e rThank du e ar s , Malay States, for placing Mr. Wilshaw's services at the disposal of the Institute in connection wit e aeriahth l survey.

Kuala Lumpur, 7th July 1937.