FUNGUS DISEASES.

New Zealand Herald, Volume LXVI, Issue 20302, 9 July 1929, Page 5

FUNGUS DISEASES.

HOW THEY AFFECT PLANT!). BY F.S.K. ' Gardeners are mostly able to recognise a fungus when they seo it and especially those that infect tho common crops, but how many know just why it is that they a.ro injurious to tho crops? Iliey know that some are merely a hindrance to the plants and others eat away tho tissues, but very few know what tho difference is. At tho commencement of its life history a fungus starts from a spore. This germinates and enters tho tissues of the leaf (or root) through a breathing pore. The thread-like fibres extend rapidly and push their way into tho inter-cellular spaces forming an internal network. All fungi are primarily parasitic, but the degree of parasitism varies with the different species. Some fill tho intcr-cellular spaces and absorb the plant juices, thus robbing the plant of a certain proportion of its nutriment.

This is bad enough, but there are other fungi that cat away the cell-contents and stop their functioning. This hits the plant in two ways and the result is that the affected portion of the leaf is killed. A typical example of a fungus of this nature is the Irish blight of the potato. The loss of a proportion of the leaf surface reduces the extent of the laboratories of the plant and the tubers caunot fill out as much as they might have. Tho network of threads i 3 known as the mycelium and when this has penetrated a certain area of the leaf tissue it sends out aerial hyphao (upright branches of the mycelium) at tho top of which are the sporangiuins in which the spores develop. The hyphae give the leaves a " fluffy" appearance, this being seen to advantage around the edges of the patches of Irish blight. On ripening the sporangiums burst and liberate thousand!' of sporos which blow about and spread the disease to other parts of tho same plant or to other plants. The spores are usually able to withstand adverse conditions for some time until a suitable host plant is encountered. «

Onco tho thread!? from the spores have entered through tho breathing part they ate in an impregnable position and little can be done to savo tho plants. This is why all methods of controlling fungus diseases are essentially of a preventive nature.

The germinating spore is the weakest stage in tb.o life history of the fungus and if it can bo destroyed then the plant is safe. It is the destruction of the germinating spore that oil the fungicidal sprays aim at. It is obvious, though, that the spray must be on the foliage of all susceptible plants before the spores alight. Early spraying when the plants aro small is recommended on this account. For any fungicide to be effective it must contain some substance which irritates the young shoot developing from the germinating spore. On looking over the various sprays it is at once noticeable that all those which are common and effective contain a proportion of sulphur or some compound of sulphur. Striking examples of this are found in Bordeaux, Burgundy, lime-sulphur and atomic sulphur and also sulphide of potassium. Sulphur apparently is caustic enough for the purpose, or else, if absorbed, poisonous enough to destroy the germinating spores. The precise effect of the sulphur is not known for certain.

Successive sprayings are essential il. the plants are to be kept free from infection for tho entire period of the maturation of the crop. At all times the whole leaf surface should be kept covered with a thin film of spray. This is why the period between successive applications should bo shortened if heavy rain is experienced. The spread of any fungous pests is hastoued by humid conditions. There arc reasons for this. First, the warm atmosphere assists the ripening of the sporangiums and tho liberation of the dangerous spores and also helps the extension of the mycelium. Secondly, the warmth and moisture supplied by tho humidity provide the two things necessary for the rapid germination of- the spores. With hundreds of spores germinating it is no wonder that infection spreads rapidly in humid weather. Extra spraying precautions should be taken while such conditions last. SPROUTING ONIONS. AN EARLY CROP. When looking over the onions in store, ono may come across some which are sprouting. These should be picked out and those which are only just beginning to produce shoots may be used. The more advanced ones are useless for culinary purposes, however, as their flavour is impaired. These bulbs arc not a total loss, though, for they may bo replanted to provide an early crop. When planted out the bulbs grow and send up seed stalks which are nipped out. just below tho flower head when it. is about to open. They mature early and produce a crop of thick-necked onions. These bulbs are useful for soups, stows, etc., at a time when the stored produce is used, and it is too soon for tho new crop. The sprouting bulbs are planted about Gin apart in soil prepared in tho ordinary way. Other than tho nipping of tho seed stalk this batch of onions requires very little attention. The routine spraying to prevent mildew will, naturally, be needed. GRAFTING FRUIT TREES. PROVISION OF SCIONS. Although tho grafting season does not come round until the sap rises, it is not too soon to make preparations for this practice. The provision of the scions is necessary before tho pruning is all dono. Suitable scions should be selected from among the prunings if possible or else taken specially from the trees. Shoots of tho right size according to the method by which tho grafting is to be dono should be selected, and a few extra allowed in case of emergencies. Grafting offers a solution of the lem caused by budding failures. Where buds failed to "take," grafts may bo tried with perhaps moro chances or success. There is no doubt as to the amount of sap flowing at the time grafting is done, whereas in tho 'case of buds it is always problematical whether there will be a sufficient flow of sap during tho remaining portion of tho autumn to ensure a successful union. It is important that tho scions bo selected only from the best trees, however, as it, is a waste of time grafting an inferior variety on to tho stock. Thoro ,aro good and bad specimens of good varieties. The fruiting characteristics are contained in every cell of every twig of a good tree, and even if woody shoots are chosen it is only a matter of encouraging the latent Truiting characteristics before one lias a Tree tho equal of the parent.. Hie scions should be heeled in in a shady place? until rcquirod.