Laying the Bugaboo of Disease 



W. H. RANKIN 



Cornell University 



DISTRIBUTION AND CONTROL OF SOME IMPORTANT PLANT PESTS— ROSY OUTLOOK FOR THEIR MASTERY- 

 NATURAL CONDITIONS THAT HELP THE GARDENER 



GROWERS often become discouraged 

 because native or newly introduced 

 diseases seem to make impossible the 

 further successful cultivation of cer- 

 tain plants. In most cases this pessimism is 

 unwarranted. In the first place destructive 

 diseases which seriously affect production are 

 usually limited to certain regions. 1 his is due 

 to the influence which temperature and mois- 

 ture play in determining the rate of spread and 

 destructive potentialities of the parasitic 

 fungi or bacteria which cause plant diseases. 

 Likewise, the variations in these climatic 

 conditions may lead to little or no loss during 

 certain seasons and extreme losses at other 

 times. It is thus possible to grow certain 

 crops with less chance of loss in some sections 

 of the country than in others. If, however, 

 the crop is grown where losses commonly 

 occur, the grower may usually apply definite 

 control measures which have been proved 

 practicable. 



THE fungus causing the late-blight and 

 rot of potatoes was brought into this 

 country many years ago, supposedly from 



Late-blight and rot of potatoes is most destructive in 

 northeastern and north central United States. The early 

 spring crop in the South is also affected frequently. Spray- 

 ing potatoes, in the northern region indicated, is a neces- 

 sary part of potato culture 



Peach leaf-curl is most common in the areas here indi- 

 cated. Generally speaking all growers in this region must 

 spray every year to avoid frequent losses 



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Ireland where it had been introduced from 

 South America. In the accompanying map 

 is shown the regions where late blight and rot 

 occur more or less destructively according to 

 the seasonal conditions. Comparatively low 

 temperatures and abundant moisture are re- 

 quired for the development of the causal 

 fungus. It is, therefore, not destructive in the 

 South except in the case of the early spring crop 

 and at high altitudes where the temperatures 

 are more like those in the North. In the 

 northern regions, however, spraying with bor- 

 deaux mixture, 5-5-50 will not only prevent 

 late-blight and rot, but will also diminish 

 losses from early-blight, tip-burn and flea 

 beetles. All these troubles are so common 

 that they do not need description. 



Begin spraying when the plants are about 

 six inches high, followed by subsequent appli- 

 cations at least every ten or fourteen days 

 until the time when the tops are killed by 

 frost. Continuous spraying is necessary even 

 though the blighting of the tops late in the 

 season may not seriously reduce the yield, for 

 the reason that serious losses from rot may 

 follow if the blight obtains a foothold. Spray- 

 ing accomplishes the best results if the mixture 

 is applied just long enough before rain periods 

 for it to become dried. It will not wash off, 

 as is generally supposed, and is present on the 

 leaves at just the time it is most needed, 

 since the fungus is distributed through the 

 field and invades the leaves while the plants 

 are wet. 



PEACH-leaf-curl has been in this country 

 for a long time and although it occurs, to 

 some extent, wherever the peach is grown, 

 it is destructive only near large bodies of water 

 and in regions with a cool climate and abundant 

 rainfall. The conditions which lead to de- 

 structive attacks are warm weather at the 

 time the buds burst, followed by a cool humid 

 period of several days. The resistance of the 

 newly exposed leaves seems to be lowered by 

 the cessation in growth due to the cool weather 

 and at the same time these weather conditions 

 are favorable for the development of the 

 fungus. The disease makes its appearance as 

 soon as the leaves unfold. The leaves are at 

 first thickened and puckered along the mid- 

 rib, causing the leaf to curl. The diseased 

 portion of the leaf is brittle and slightly yel- 

 lowish, tinged with red. The affected leaves 

 finally drop from the tree and new leaves come 

 out later from dormant buds. The twigs 

 may also become affected. 



One application of any common fungicide, 

 at any time after the leaves fall in the autumn 

 (and before the buds burst in the spring) 

 will prevent this disease. Every twig and 

 bud must be coated with the fungicide in order 

 to destroy the over-wintering parts of the 

 fungus. Lime-sulphur, 1 gallon to 8 of 

 water, bordeaux mixture at any common 

 strength, and copper sulphate 2 pounds to 

 50 gallons of water, serve equally well for the 

 control of this disease. 



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Apple-rust is common in certain portions of the area 

 shown on this map. Many of the other similar rust dis- 

 eases of Juniper, pear, quince, Haw, Sef%ice-berry, and 

 Mountain Ash occur in the same region 



[ANY of the native rust fungi which 

 cause important* diseases of orna- 

 mental conifers and pomaceous shrubs and 

 trees (apples, etc.) are controllable by separ- 

 ating the two kinds of trees which these fungi 

 require for the completion of their life-history. 

 Several of the rust diseases of the cultivated 

 apple, pear, quince and Mountain Ash, and 



28 



ornamental species of Haw and Service- 

 berry require the presence of certain species 

 of Juniper or Red Cedar in the immediate 

 locality, otherwise the rust fungus can no 

 longer perpetuate itself. Serious defoliation 

 of the pomaceous trees results when Red Ced- 

 ars are in close proximity and the Cedars them- 

 selves may be deformed by the production of 

 cedar-apples, witches' brooms, knots, galls and 

 long spindle-shaped swellings of the trunks 

 and branches. The spores formed on the 

 deformed parts of the Cedar cause the in- 

 fection of the leaves, twigs and fruit of the 

 trees of the apple family and in turn the spores 

 from these trees cause the infection of the 

 Cedar. Spraying the Cedars or the pomaceous 

 trees has not been found practicable. On the 

 other hand by separating these two kinds of 

 trees by a distance of a half mile or more the 

 chances for the exchange of spores between 

 them is reduced to the point where the dam- 

 age they may cause is unimportant. 



CEVERAL rust diseases of Pines are 

 ^ similar in nature, in that the associa- 

 tion of a certain other kind of plant is Re- 

 quired else the fungus cannot exist in the 



The fungus causing the blister rust of White Pine is 

 now known to be prevalent in the areas here shown. The 

 stages on the gooseberry and currant have been found 

 abundant. Important infestations of Pine forest areas are 

 known in the New England States and New York 



Bean anthracnose occurs destructively in the northeastern 

 and southeastern parts of the United States. In the South 

 only the early spring crop is affected. Pod selection will 

 yield clean seed and reduce losses materially 



The chestnut blight fungus has now spread to practically 

 the entire area where the chestnut grows abundantly 



