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THE OCCURRENCE OF PERITHECIA OF THEOAK MILDEW IN BRITAIN

By NOEL ROBERTSON AND IAN MACFARLANE

During a foray to Bricketwood, Hertfordshire, on 6 October 1945, sixperithecia (cleistocarps) of the Oak Mildew were found on a living leaf ofQuercus Robur L. Many living leaves bearing the mildew were examined,some from the same tree and some from other trees nearby, but no moreperithecia were found. This collection, like those of other powdery mil­dews hitherto found on oaks, proved to belong to the genus Microsphaera,which is distinguished from the other genera ofErysiphaceae by the presenceof more than one ascus in the perithecium and by the characteristicdichotomous branching of the tips of the appendages. The mycelium wasabundantly developed, mainly on the adaxial surface of the leaf, and boreelliptical to barrel-shaped conidia measuring 25-37 x 15-22fL with anaverage size of 31 x 19fL. The perithecia were 180-200fL in diameter, theappendages, twenty to twenty-four in number, were from 170 to 300fLin length and the spores (twenty from three perithecia) measured 18-24 x6-13 fL ·

The perithecia of the Oak Mildew have been recorded from manyEuropean countries at infrequent intervals (Blumer, 1933), the earliestrecord being that of Arnaud and Foex (1912) from south-west France in191 I. The perithecia have only recently been recorded from those countrieswith a maritime climate: they were found in Norway in 1932 (jorstad,1945) and in Holland in 1939 (Hartsuijker, 1939), and have still to berecorded from Sweden and Belgium. It is generally believed that the de­velopment of the perithe cia is inhibited in a maritime climate, whereastheir development is encouraged by periods of hot, dry weather. Laibach( 1930), using potted plants, has shown that by keeping the soil dry, or byreducing the relative humidity of the surrounding atmosphere (or by com­bining both treatments) he was able to induce the production of perithecia.Conditions at Bricketwood seemed dry for the time of the year, and thenumber of terricolous Hymenomycetes was abnormally low according tothose who had visited the wood in previous autumns. Weather recordscollected at Rothamsted (eight miles away) showed that the total rainfallin the thirty-three days from 4 September to 6 October 1945, was abouthalf (I .113 in.) that of the average for the same period in the ten previousyears (2'25 in.), but, in the absence of extensive data on the subject, we donot wish to imply any causal relationship between the dryness of theseason and the occurrence of the perithecia.

We have identified the fungus provisionally with Microsphaera alphitoidesGriff. & Maubl. (1912), since it agrees closely with the description of theoriginal authors and also with that of Blumer (1933). We have matchedour specimen with the exsiccatus no. 3099 of Sydow's Mycotheca Germanica,issued as M. alphitoides, and find that it agrees closely. The average diameterof the perithecia collected at Bricketwood is 180fLcompared with Blumer's

220 Transactions British Mycological Society

report of I 16fL (range I03-130fL), but the other characters agree, and wethink that we are dealing with the same fungus. If this variation in differentcollections is caused by environmental conditions, it is possible that theperithecial diameter should not have the importance given it by Blumerin his delimitation of species. We have seen one collection of M. densissima(Schw.) Cooke & Peck (collection by Gerard from the vicinity of NewYork, in Herb. M. C. Cooke, 1885 in the herbarium of the Royal BotanicGarden, Edinburgh), from which it is difficult to differentiate M. alphi­toides on morphological grounds alone, although the distal dichotomy ofthe appendages in M. densissima appears to be more elaborate. This lastcharacter is a variable one, however, and we have observed some peritheciain which the appendages were very similar to those of M. alphitoides (ourown specimen and Sydow's no. 3099), It may be worth noting that thetype collection of M. alphitoides Griff. & Maubl. is the perithecial materialcollected by Arnaud and Foex and identified by them with the Americanspecies M. quercina (Schw.) Burr. Further study of the variation of theAmerican Oak Mildews may show that the European form could be in­cluded as one variant within a complex. Since the first discovery of itsperithecia in 191 I the descriptions and collections of the Oak Mildew inEurope have been consistent, the fungus is distinctive among the oakmildews in the massive development of a thick, white mycelium and, as faras is known (see Neger, 1915), it is more or less confined to the Europeanoaks, with records on the beech (Fagus sylvatica L.) and the chestnut(Castanea sativa MilL). This combination of uniformity with host rangeand geographical distribution suggests that it is useful to maintain Micro­sphaera alphitoides as the name of the European Oak Mildew, until furtherinvestigation of the variability and host range of the American oak mildews,which have been classified differently by almost everyone who has studiedthem, finally settles the question of its identity.

Permanent mounts of single perithecia have been made and depositedin the British Museum (Natural History), the Herbarium of the RoyalBotanic Gardens, Kew, the Herbarium of the Royal Botanic Garden,Edinburgh and in the Cambridge Botany School.

REFERENCES

ARNAUD, G. & FOEX, E. (J9I2). Sur la forme parfaite de I'oidium du Chene. C.R. Acad,Sci., Paris, CLlV, 125.

BLUMER, S. (1933)' Die Erysiphaceen Mitteleuropas. Beitr. Kryptogamenfl. Schuieir,p·3 I6.

GRIFFON, E. & MAUBLANC, A. (1912). Les Microsphaera des Chenes, Bull. Soc. mycol.Fr. XXVIII, 88.

HARTSUIJKER, K. (1939). Perithecien van den Eikenmeeldauw: Microsphaera quercina(Schw.) Burr. Tijdschr. Pl;:,iekt. XLV, 162-5.

JORSTAD, IVAR (1945)' Parasittsoppene pa Kultur--og Nyttevekster i Norge. Meldingfra Statens Plantepatologiske institutt, Nr. I, 34.

LAIBACH, F. (1930). Uber die Bedingungen der Erysipheen, ]b. wiss. Bot. LXXII, 106-36.NEGER, F. W. (1915)' Der Eichenmehltau (Microsphaera alni Wallr. var. quercina).

Naturui. Z. Forst- u, Landui. XIII, I.

(Acceptedfor publication 6 July 1946)