Letters on the Diseases of Plants. 



33 



Preventive Measures. 

 4. Do not buy trees except under guarantee that they are perfectly 

 healthy. Nurserymen shoiold he terij careful not to take scions from peach- 

 trees that have shown the disease. If this measure could be enforced there 

 would be very little of this disease. 



3. Peach Rust. 



Eust is the very common disease that in the autumn attacks the leaves of 

 peach, plum, and apricot trees, but especially those of the peach. As a result 

 the leaves turn yellow and fall off soouer than they otherwise would, and 



Fig. 39,— Germinating spore of the peacli-rust fungus, Piiccinia pnirii, Persoon. 



Uredo stage. — The yellowish, or brownish ovate to pyriform finely 

 echinulate iiredospores are borne on pediceles among numerous trans- 

 parent capitate paraphyses in round and raised pvilvenilent light-brown 

 sori, which occur in crowded groups on the under surface', of the leaves 

 and young twigs of the host-plant, generally giving rise to a yellow 

 discoloiiration. The apex of the spore is recognized at once by its 

 darker colour and thicker wall. The pediceles are aboiit twice as long 

 as the spores. On germinating each spore displays threejgerm i^ores, 

 but gives rise, however, to only one hy[jha. Germinating spores average 

 16 X 37 /U, hut vary from 12 x 42 to 17 x 22. The sori measure three- 

 tenths of a millimetre in diameter, but several may run together, giving 

 rise to larger compoimd sori. On the fruit of the peach the sori are 

 commonly small (punctate) and scattered. 



Fuccinia stage. — The pedicellate teleutosiiores, measuring? 17 'x 31yU. 

 are composed of two nearly spherical cells — a larger terminal dark-brown 

 cell, and a smaller basal cell of a lighter hue — both being flattened at the 

 point of union, and both presenting nivmerous short but stout spines. 

 The pediceles are of about the same length as the spores. The cell 

 wn.lls are of uniform thickness. The dark brown or black round piUveru- 

 lent sori are situated as in the case of the uredo sori, and measure one- 

 foiu-th of a millimetre in diameter. 



Hab. — Leaves, young branches, and fruit of the peach, wherel it | gives 

 rise to mvich troiible, causing the leaves to be shed prematurely, 

 thus diminishing the vitality of the tree, as well as giving the fruit an 

 unsightly appearance. This species is the JJi-omyces amygdali of Cooke, 

 of which original specimens have been presented to the Department by 

 Mr. Bailey, the Government Botanist, Queensland. We do not see that 

 it differs in any marked way from P. prwni, Pers., and in the absence of 

 any cultivation exxjeriments must regard the two as identical. The spores 

 are devoured by the larva of a species of DipJosis. 



the growth of the tree is impeded. The appearance of the disease need 

 hardly be described to owners of peach-trees, it is so very common. Suffice 

 it to say that the rust is due to a fungus belonging to the same family as 

 the common and notorious wheat rust, that it occurs abundantly on the 

 lower side of the leaves in the shape of small pustules, each of which gives 



Fig. 40.-Portion of a peach leaf near the mid-rib as 

 seen vmder a magnifying-glass. Pustules, due to 

 the peach-rust fungus, are shown, and, growing 

 in the midst of them, the dark pycnidia that 

 give rise to the two-celled spores shown in Fig. 44. 



Fig. 41.— Spore of 

 the peach-rust 

 fungus magnified 

 more than in Fig. 



Pig. 42.— Teleutospores of 

 the rust-fungus found at- 

 tacking the leaves of the 

 plum-tree. 



rise to a brownish powder, which, when examined with a microscope, is 



found to be composed of bodies shaped like those shown in Figs. 39 and 41. 



I have made in connection with the fungus causing this disease a number 



of observations that are of some considerable interest from a scientific point 



c 



