74 



IRISH GARDENING. 



Sclerotinia Diseases- 



TilK diseases dealt within this artiele cover thoso 

 caused by the fungus Sckn'otinia scl3rotioruni, 

 Bref., a parasite well known both in Eiirope and 

 America, and capable of attacking a large range 

 of cultivated plants, l)oth annual and herbaceous. 

 The method of infection aiid behaviour of the 

 fungus have 1 een most carefully studied in the 

 case of n disease of the jjotato whicli is wi(h'ly 

 distributed in Kngl nd and Scotland, and causes 

 serious damage in Ireland. As the life-history 

 of the parasite and the treatment to be adopted 

 are the same when other crops are concerned, the 

 potato disease only is described in detail. 



S. sclerotiorum is allied to S. trifolioruni, one 

 of the fungi associated with clover sickness and 

 described in Leaflet No. 271, and also to S. 

 bulborum, a fungus which attacks varioas 

 ])ulbous plants in gardens. 



The Stalk (Sclerotinia) Disease op Pota- 

 toes (Sclerotinia sclerotioritm, Bref.). — 

 The stalk disease of potato is most destructive in 

 the northern and damper parts of the country. 

 In the west of Ii-eland th(^ loss occasioned by it is 

 so great that, with the exception of the ordinary 

 potato blight (Pliytophthora infestans), it is 

 stated to be the most serious disease with which, 

 growers have to contend. The fungus attacks the 

 stem, either near the ground or at some distance 

 above it. Subsequently it penetrates the inner 

 tissues and destroys them so that the stem falls 

 over at the affected spot and dies. Though in 

 this disease the tubers are not attacked the yield 

 is reduced owing to the death of the shoot, and in 

 districts where Sclerotinia is widespread the cro]i 

 may suffer very severely. 



Descripikni and Life-History. — In the earliest 

 pliases of attack, usually about the beginning of 

 July, the disease shows itself in the form of white 

 patches of fungus threads or mycelium, on the 

 outside of the stem. In contrast to most stem 

 diseases this is accompanied by but little yellow- 

 ing of the foliage, with the result that infected 

 plants are easily ovei'looked. If dull, dami) 

 weather prevails the mycelium develops rapidly 

 and begins to forni oval or spherical cushions, 

 white in colour, and from which minute drops of 

 water exude. These cushions represent the 

 youngest stages of the resting bodies known as 

 sclerotia. The sclerotia consist of a compact 

 mass of mycelium, which later becomes firm and 

 Anally hard and black, though internally it 

 remains white. They are spherical or oval in 

 shape, and usually about the size of a pea, but 

 frequently much elongated. When ripe they fall 

 off and remain dormant in the soil until the 

 following spring. It is from the possession of 

 these sclerotia that the fungus derives its generic 

 name Sclerotinia, though it should be remem- 

 bered that sclerotia are also produced by many 

 other fungi. 



In addition to forming external mycelium and 

 sclei'otia the fungus gradually penetrates the 

 inner tissues of the stem. The cells are invaded 

 and the pith-cavity is filled up with fluffy white 

 mycelium in which sclerotia, similar to those 

 produced externally, develop. The latter remain 

 inside the stems, but ultimately reach the soil if 

 the stems are allowed to decay on the land. The 

 result of this internal development of mycelium is 

 the blocking up of the water-conducting chantrels. 



At the point of attack the tissues are killed and 

 the stem Ix-nds over, and sooner oi' later dies. 



'i'iie faiv of tin- sclerotia in the soil has been 

 caicfuliy studi<Ml by several observers. They 

 remain dormant until early summer, when they 

 germinate and give rise; to snuill disc or cup- 

 shaped bodies which i)roduce tiie spores. The 

 cups are ))or)ie on slender stalks, and appear just 

 abo^ e the surface of the soil. They are pale 

 brownish-yellow in ci^lour, and from one quarter 

 to one-half an inch in diameter. These disc- 

 shaped cups are termed apothecia, and they form 

 a characteristic feature of the very large group 

 of fungi known as Discomycetes. When ripe the 

 apothecia discharge their spores into the air, 

 usually in large numbers at a time. If the 

 apothecia are carefully watched, smoke-like puffs 

 of spores may easily be seen. The intermittent 

 discharge of spores from a single cup may 

 continue for two or three weeks. 



The infect"on of the potato plants by the 

 fungus was previously thought to take place by 

 means of vegetative mycelium present in the 

 soil, but recent investigations carried out in 

 Ireland have shown that this is not the case, but 

 that infection is brought about exclusively by 

 air-ljoiTie spores derived from the apothecia. 

 The spores are Idown across the fields and alight 

 on the foliage. On germination they are capable 

 of infecting the older and fading leaves, and 

 from the leaf the fungus passes into the stem. 

 In some cases diiect infection of healthy tissues 

 apparently also takes place, especially in such 

 spots as leaf axils where moisture is preserved. 



S. sclerotiorum possesses no conidial form of 

 reproduction. The Botrytis found on potato 

 haulms, and formerly thought to l)e a stage in the 

 life cycle, is now known to be an entirely distinct 

 fungus. In winter, sclerotinia is perpetuated 

 l)y the hard, black sclerotia in the soil, and in 

 early sunimer it is propagated by means of the 

 spores liberated from the cup-shaped apothecia. 



Treatment. — (1 ) The most important measure 

 to adopt is systematically to collect and burn all 

 diseased portions of the plant in order to prevent 

 the sclerotia from reaching the soil. If this is 

 carried out thoroughly the number of spore- 

 bearing cups produced i'n spring will be largely 

 reduced. 



(2) Ti-eatment of the soil with lime in order to 

 kill the sclerotia has not jjroved of any value, 

 and experiments on spraying plants and soil with 

 fungicides, with a view to killing the spores and 

 sporescups, have not yielded satisfactory results. 



(3) For greenhouse or garden work sterilisation 

 of the soil by steam may be recommended. 



(4) Unless the soil has been sterilised, plants 

 liable to be attacked, by sclerotinia should not be 

 grown for at least three years in infected soil. 

 The fresh site selected shoidd be well removed 

 from the old one. 



(5) In the case of potatoes, in the west of 

 Ireland, late planting has proved successful, the 

 explanation of the greater immunity of late crops 

 being that fewer old leaves (which provide the 

 fungus with an easy means of entry) are available 

 at the time of the main spore-discharge. 



(6) When root crops are concerned, the greatest 

 care should be exercised as to storage, and all 

 diseased or damaged roots should be rejected.- 

 Jovnial of the Board of Agriciillnrc, February, 1917 



