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THE AGRICULTURAL XE^VS. 



Maech 28, 19U. 



FUNGUS NOTES. 



DISEASES OF SWEET POTATO. 



The sweet potato seems to be little troubled by fungus 

 diseases in the West Indies. It is possible that some may 

 have escaped notice or have had their results ascribed to 

 insects, but so far as the writer is aware only the white rust 

 of the leaves due to one or more species of Cystopus. and 

 a certain amount of white mycelium occurring on the roots 

 and attributed to J/arasmiits sp. have attracted attention. 



In the United States, the li.st of diseases appears to be 

 a formidable one. and a number of papers on the subject have 

 lately appeared. In view of the desirability of collecting 

 more information as to sweet potato diseases in these islands, 

 the following summary, compiled from various American 

 sources, of the characteristics of the diseases occurring in that 

 country is given. 



FOOT ROT. (Plenodomus destruens. Harter). 

 This disease occurs in Virginia. 



Inquiries among farmers make it appear probable that 

 it has increased in severity in the past few years, so that 

 it is likely to prove a serious handicap to the industry. 



The first visible sign of the disease is a blackening of the 

 lower part of the stem followed by the yellowing and dying 

 of the lower leaves. The cortex of the stem is turned black 

 from a little below the soil line to 3 to 5 inches above it. 

 Shortly after this has occurred the plant wilts and gradually 

 dies. Pycnidia appear previous to this time on the blackened 

 area, and continue to appear for some time afterwards if 

 sufficient moisture is present. The disease does not occur on 

 the leaves, but it is able to invade and rot the potatoes. It 

 ias also been occasionally found, in low wet fields, on the 

 vines several feet from their point of origin. 



The disease is due to a fungus described as new under 

 the name of Plenodonms destruens; it is closely related to 

 Diaportlie [Phomopsis, Phoma] hatatis, which causes the 

 dry rot of the sweet potato, but differs from it in some 

 morphological characters and has an entirely different habit 

 of growth and parasitism. The pycnidia are at first buried 

 in the substratum, but later break through the epidermis, 

 appearing as black dots scattered over the surface. They 

 are irregular in form and vary greatly in size. 



Inoculation experiments from pure cultures were 

 successful with spores and with hyphae when introduced 

 into wounds or smeared on the uninjured stems. Spores 

 suspended in sterile water were poured about ten plants, 

 seven of which died of the disease. Plants at all ages were 

 about equally susceptible. 



No ascogenous stage of the disease was found, but it 

 ■was proved that the disease can persist through the winter 

 on dead vines and that stored potatoes can carry the disease. 

 It would appear that the control of the disease will depend 

 upon careful sanitation and the rotation of crops, combined 

 with strict precautions against the planting of diseased 

 material. 



BEY KOT {Diaportlic. hatatis, (E. k H.) Hart. & Fid.). 



This disease has been known since 1890 under the 

 name of Phoma hatatae, but recently the ascogenous stage has 

 been found to belong to Diaporthe, while the pycnidial stage 

 is assigned to Phomop.'^is. According to the same account 

 the disease is more de.<<tiuctive than is generally supposed, 

 its efiects in the field being often attributed to other causes. 

 It causes a rotting of tbe"stems of the vines, but is chiefly 

 known as an affection of the stored tubers. These com- 

 mence to shrivel from the stem end, and eventually become 



quite dried up. The pycnidia appear in little pustules over 

 the surface. The disease is mainly distributed by the 

 'seed' and by the presence of decayed potatoes in manure. 



BLACK EOT (Sphatronema iimhriatum, Sacc. [C'eratocpstis- 

 iimhriata, E. it H.]). 



This disease produces sunken greenish-black spots or 

 blotches, with olive green tissue below them, on the stems 

 and roots. On the full-grown root these may come to cover 

 practically the whole surface. When uncomplicated by 

 other organisms it remains superficial, but unless the spot& 

 are cut out before the potato is cooked they impart a bitter 

 taste to the vegetable. There are several kinds of spores 

 produced: one kind formed similarly to the endoconidia of 

 the pineapple disease of sugar-cane, another kind which 

 consists of thick-walled unicellular conidia — borne singly, by 

 twos, or in chains — and finally, long-necked pycnidia occur 

 in the diseased spots and extrude minute globular spores in. 

 a gelatinous mass. 



CHARCOAL EOT (SderotiuiH hataticola, Taubenhaus). 



lentil recently this fungus was considered to be a stage- 

 of the black rot fungus just described, but has now been 

 determined as distinct. It does not seem to produce surface 

 spots but invades the entire contents of the root, altering 

 the tissue to a charcoal-like mass. The blackening is 

 due to the formation of numerous very minute jet-black 

 sclerotia. The fungus is parasitic in its nature. 



.JAVA BLACK EOT (Lasiodiplodia tuLericola, E. A- E.). 

 This fungus appears to have been introduced into the 

 LJ^nited States from Java. It is an active parasite and causes 

 dark shrivelled patches on the potatoes. Numerous pycnidia 

 are produced, which emit one-septate dark spores, heaped 

 together, or white strings of unincellular hyaline spores. 

 The fungus attacks the interior tissues from the point and 

 turns them jet-black. Infected roots dry and shrivel and 

 become brittle. 



s-oiL EOT (Acroct/stis batatae, E. &, H.). 



This is reported to be one of the most serious of the 

 diseases of sweet potato, sometimes causing almost total loss 

 of crop. It is strictly a field disease, affecting the roots at 

 an early stage. Growth cease.s at the points of attack, result- 

 ing in the production of constrictions, between which the 

 potato swells out, so that it comes to have the appearance of 

 an hour glass or of a string of small potatoes. The spores 

 of the fungus contaminate the soil, and long rotation, or 

 treatment with kainit and sulphur, 300 to 400fl). of each per 

 acre, is recommended. 



SOFT EOT {Rhi.opus nigricans, Ehr.). 



This fungus is one of the cosmopolitan species of 

 moulds, and is liable to cause a soft and putrid decay of the 

 stored roots, especially when they are sweating. 



SCCEF {Monilioehaetes in/uscans, E & H.). 



A brown and rnsty coating is produced on the surface 

 of potatoes in the field by this fungus, and it continues to 

 spread on the stored roots. It causes shrinking, but does 

 not result in decay. 



STEM EOT {Nectria ipomueoe, Hals.). 



This appears to have a general resemblance to the foot 

 rot described above. The vine near the surface of the ground 

 turns first yellow and then black, and dies while still 

 young. The rot also spreads to the upper part of the root. 

 It is a field disease and does not attack stored potatoes. 

 A creamy white mycelium is found near the surface of the 

 ground, producing i. Fusarium type of spore. Later the 

 perithecia appear in clusters at the base of the stem. The 

 same fungus attacks the egg plant. 



