October, 1411. 



KNOWLEDGE. 



407 



Figure 1. A Feacli branch 



attacked by peach curl, Exoascits 



dcfoniians. 



l-lc.L Kl. 2. Vcilital Mcliiiii >A a 



leaf of Peach infected with Exoascits 



dcfonnans, X 500. 



EXOASCLS DEFORMANS BERK. 

 (PEACH CURL).— This fungus is the 

 cause of a remarkable disease in Peach 

 trees often proving fatal to young stock. 

 Although the mycelium is perennial in the 

 branches, the disease first becomes evident 

 in early spring when the trees are producing 

 their first leaves. Many of the leaves are 

 at once virulently attacked and as a result 

 they assume the remarkably thickened and 

 curled appearance as seen in the first 

 photograph. In colour they are yellowish- 

 green splashed with red, probably due to 

 the decomposition of the chlorophyll. As 

 a consequence of this, photosynthesis is 

 inhibited, resulting in a poor development 

 of the plant. .After a period of vegetation 

 the fungus reproduces itself asexually. On 

 certain localised areas of the leaf minute 

 club-shaped bodies appear. These organs 

 are asci, a reproductive structure typical of 

 the large order of fungi — the Ascomycetes — 

 of which Exuasciis is the simplest represen- 

 tative. The contents of each ascus di\'ides 

 to form eight spores — ascospores, which 

 again undergo division resulting in the 

 formation of a large number of minute spores 

 (see photograph) which on liberation carry 

 on the work of destruction. Owing to the 

 fact that the mycelium is perennial the 

 disease is not easily eradicated. The usual 

 method adopted by the nurseryman is the 

 picking off and burning of all infected 

 leaves. 



CYSTOPUS CANDIDUS PERS. 

 (WHITE RUST) ON CAPSELLA 

 BURSA PASTORIS (SHEPHERD'S 

 Pl'RSE). — Figure 3. Cystopus candidns 

 is an interesting member of the Oiimycetes, 

 a sub-class of the Phycomycetes or algal 

 fungi — a name given to the group on account 

 of the structure of the se.xual organs, which 

 are strikingly similar to those of the 

 Siphonae, of which N'aucheria is an 

 e.xcellent type. The Phycomycetes arc 

 considered to be derived from the Siphonae. 



The white patches well seen in the 

 photograph are the sporangia of the fungus, 

 the structure of which is demonstrated in 

 Figure 4. The mycelium ramifies through- 

 out the tissues of the stem, leaf and fruit, 

 and eventually breaks through the epidermis 

 giving rise to pustules of sporangia. These 

 sporangia are produced in chains and on 

 germination give rise to numerous swarm 

 spores, which, given moist conditions, 

 rapidly infect neighbouring plants. The 

 sexual organs are antheridia and oiigonia 

 — arising icitliin the host plant. The 

 oiigonia are spherical swellings fonned at 

 the ends of the hyphae, or intercalated 

 throughout their length. The contents of 

 the oogonium become differentiated into 

 an oospore and peripheral periplasm. The 

 antheridia are tubular outgrowths, cut off 

 from the parent hypha by a cell wall. 

 .\fter fertilisation the oospore surrounds 

 itself with a thick wall of a brownish 

 colour, the surface of which is covered 

 with irregular warts. After a period of 

 rest the oospores germinate. In the hard 

 spore walls a fissure appears, and the 

 contents of the spore, surrounded bv 

 the inner membrane, are pressed through 

 the orifice. The protoplasm divides 



FlGURli 3. Stems of Shepherd's 



Purse attacked by white rust, 



Cysfupiis caiididns. 



IGUKl-J 4 



IGUKl-J 4. \ I itiial secticin nt tlic 

 stem of Shepherd's Pur.se infected 

 with ('vstnbiis Ccliulidn ■ ^ ■""' 



item ot Shepherd s Pur.se mtectei 

 witli Cystottiis ciiiultdiis, X 200. 



