318 



THE AGRICULTURAL NEWS. 



September 30, 1911. 



FUNGUS NOTES. 



BLACK ROT OF NATAL CITRUS FRUITS. 



Some interesting results, arising from an investigation 

 into tlie cause of a black rot of various citrus fruits in Natal, 

 have recently been published in Science Bulletin No. 1, of the 

 Transvaal Department of Agriculture, entitled On the Struc- 

 ture and Life-History of Diphdia natalensis, n. sp., by 

 I. B. Pole Evans, M.A., Plant Pathologist to the Transvaal 

 Department of Agriculture. As this fungus is somewhat 

 similar to the West Indian die-back fungus of cacao, some 

 account of it and of its effects on the fruit may l)e of interest. 



EXTERNAL SYMPTOMS OF DisE.A.SK. The disease first ap- 

 peared on lemons shipped to the Transvaal from Natal, but 

 subsequent investigations showed that it could occur on all 

 forms of citrus fruits. The account of its symptoms, as given 

 by Pole Evans, is as follows: — 



' The first evidence of disease is a translucent or 

 watery appearance of the rind, usually around the stalk 

 end of the fruit. This is quickly followed by a soften- 

 ing, and gradual brown discoloration of the affected tissue. 

 The brown discoloration, when it has once appeared, 

 spreads very rapidly and uniformly over the whole fruit, 

 which then becomes distinctly sticky to handle, vv'hile at the 

 .same time a greenish-brown liquid exudes from it where it 

 comes into contact with anything on which it is resting. Very 

 soon after this a dark olive-green to black discoloration appears 

 at the stalk end, and from thence encroaches over the whole 

 fruit, until it is converted into a black, mummified mass, with 

 a very wrinkled and shrivelled surface. 



' Fruit in this condition, if left exposed to a dry atmo- 

 sphere, soon dries out, and remains indefinitely a hard body, 

 e-icceedingly light in weight, but retaining to a considerable 

 e.xtent its original form and shape. Tlie loss of weight in the 

 fruit can be detected almost as soon as the brown discolora- 

 tion appears. When a dry lemon is cut open, the whole of 

 the pulp has become absorbed, and nothing remains but a dark, 

 fibrous mass of tissue. 



'The pips and radiating sei)ta are usually covered with 

 a dull greyish growth. t)n examining the external black 

 discoloration more closely with the naked eye or with a hand 

 lens, it is seen to be due to the formation of a number of 

 small black patches under the ei)idermis of the rind. As 

 these dark masses increase in number they run one into 

 another and coalesce, so that the whole surface takes on a 

 homogeneous dark colour. 



'If affected fruit is kept under moist conditions, a very 

 different state of affairs occurs. The surface very .soon be- 

 comes studded with innumerable tufts of hyphae, at first 

 greyish-olive in appearance, but which later on turn darker 

 coloured and then eventually conqiletely invest the fruit with 

 a dark felt like mycelial growth.' 



cau.se ok TiiH DISEASE. The disease is due to a species 

 of Dliilodia, believed by Pole I'Aans to be different from any 

 previously described, to whieh he has given the name 

 Diplodia nataloi.^ix. The fungus produces scattered pycnidia 

 beneath the rind of infected fruits. The spores are biseptate, 

 dark olive-brown in colour, and very similar to those of the 

 cacao dieback fungus, except that they are marked with 

 bands running along their length. In artificial cultures, 

 under damp conditions, the pycnidia are produced in small 

 hummock like protuberances covered with a thick felt of dark 

 olive-green hairs; this is similar to the manner in which 

 those of Tht/vidatia tmrla ate produced under similar condi- 



tions. The resemblance between these two fungi is strong, 

 though there are some points of difference. 



iNFECTiox EXPKKiMENTs These left no doubt that 

 the fungus Diplodia natalensis was the cause of the disease 

 described. They further showed that the fungus was capable 

 of living on apples and peaches, and of producing character- 

 istic pycnidia upon these at the end of ten days, although 

 there was no formation of these organs on infected lemons at 

 the end of fourteen days. In fact on lemons and oranges, 

 pycnidia only occur on the black sclerotial bodies found in 

 the infected fruit after such bodies have pas.sed through 

 a resting period of two months. If after this, the fruits are 

 exposed to the right conditions of temperature and moisture, 

 pycnidia will develop. 



Another set of inoculation experiments on lemons gave 

 rise to some points of interest. It was found that infection 

 occurred on placing spores of the fungus upon the stalk 

 scar, both when this was punctured with a sterile needle 

 and when it was left untouched. Furthermore, it was obser- 

 ved that spores placed on a slightly wounded or abraded sur- 

 face of the rind could cause disease, but that they could not 

 do so when the rind was intact. Another point revealed by 

 the experiments was that in many eases there was an incu- 

 bation period of from ten to fifteen days between the date of 

 infection and that of the first ap[)earance of the disease. 

 Thus fruit apparently quite sound might be packed in a case 

 and develop the disease in transit. 



PREVENTIVE MEASURES. The chief source of infection is 

 the mass of spores formed in the pycnidia arising on mummi- 

 fied fruit left lying about the orchards. The principal means 

 of reducing the prevalence of the disease must, therefore, lie 

 in destroying, by heat, the fungus contained in all such mumi- 

 fied fruits, which must be carefully collected and removed 

 from the orchards for the purpose. 



A fungus very similar in appearance and possibly 

 identical with that found in Natal has recently been shown 

 to cause gumming of citrus and peach trees in Florida. An 

 account of the inoculation experiments carried out to deter- 

 mine this was published in J/i/colof/ia, Vol. Ill, p. 151, 

 under the title A Gum-Inducing Diplodia of Peach and 

 Orange, by H. S. Fawcett and O. F. Burger. It was shown 

 by cross-inoculation that the fungus on citrus caused 

 gumming on peach trees, and vice versa. The same 

 fungus also occurred on rotting fruits of orange and 

 grape fruit, while inoculations witli it caused softening and 

 decay of oranges, lemons and apples, in the space of one 

 to two weeks. In the case of the two former fruits, it was 

 only necessary to place the mycelium on the stalk end to 

 induce decay, while the fungus could subsequently be reco- 

 vered from the disease<l fruit. The authors are of ilie opinion 

 that the Diiilodia causing the gumming may well ba the .same 

 as D jtatalenais,- though the point is not yet establishe 1. 



An interesting saprophytic fungus of frequent occunence 

 in the West Indies is J/inteola pii/i/lric/ia, Mont., closely 

 related to the Jew's ear fungus, found on the elder, in 

 temperate countries. The fructifications of the local 

 species are roughly ear-shaped, and are attached to the 

 substratum by a very short stalk. They are of a gelat- 

 inous consistency when moi.st, but become rigid and horny 

 when dry. The upper surface is blue, and jiossesses a bloom 

 like that of purple grapes. The surface underneath is greyish- 

 brown, and very hairy. The fungus is common on dead caeaOi 

 on wooden posts, and on all dead wood. It occurs on Para 

 rubber in Ceylon, and has been seen on Barbados evergreen 

 (Ficus sp.) killed by Euti/pa enimpem, Massee. 



