A PAAY-PAW LEAF-SJ>()T ('AT^SED JVY A 

 FHYLLOSTICTA SP. 



By Paul A. van uer Bijl, M.A., D.Sc, F.L.S., 

 Natol Herbarmiii, Dnrhan. 



Read Jul,) 15, 1920. 



A leaf spot disease does not appear to have been generally 

 reported, on the paw-paw from the Union of South Africa, though 

 it is not uncommon in Natal. 



The fungus responsible is a species of PliyUosticta. 



The discoloured spots measure on an average 2 mm. tO' 5 mm. 

 across, and may be circular or angular, or vaore or less elongated 

 in one direction. They are frequently circumscribed by a yello'.\- 

 or brownish ring, which passes into- the normal green of the leaf. 

 Individual spots frequently coalesce. On the upper leaf siu-face 

 small black dots are evident in these discoloured areas, and they 

 are the pyenidia of the causal fungus. 



The leaf tissues in the infected areas become brittle, and 

 ultimately fall out, giving the leaves a " shot-hole " appearance. 



The fungus was isolated by ordinary methods from leaves 

 obtained in plantations, and from subcultures of this paw-paw 

 leaves were artificialh' inoculated as follows: — 



Experiment I. — A small plant was taken and areas marked 

 with a pencil on the lower surfaces of three of the leaves. Pieces of 

 the mycelium froin a pure culture were placed on these areas. 

 The vmder surfaces of the leaves were then syringed with distilled 

 water, and the whole plant covei*ed with a bell jar to' preserve a 

 moist atmosphere. 



The results were as follows: — 



The first leaf had eight areas inoculated, of -which six became 

 infected after six days. The second leaf had eight areas 

 inoculated, of which six showed infection after six days. The 

 third leaf had seven areas inoculated, of which six were infected 

 after six days. 



The infected spots were circular, grej'ish or dark brown, 

 2 mm. to 7 mm. diameter, and decidedly brittle. Pyenidia were 

 evident within twenty days in the discoloured areas on the upper 

 leaf surface. 



Experiment II. was carried out in the same way, three 

 marked areas being inoculated. Two of these became infected 

 after five days, and pyenidia developed within twelve days. 



It should be noted that the period of six days stated for the 

 development of pyenidia must not necessarily be taken as the 

 minimum period, as the plants could not always be examined 

 regularly. 



