Citrus Verrucosis and Bark Blotch

New Zealand Journal of Agriculture, Volume 87, Issue 4, 15 October 1953, Page 364

Citrus Verrucosis and Bark Blotch

PRACTICALLY all kinds of fruit grown in New Zealand are subject to diseases of some description, and in that respect citrus fruit is no exception. Fortunately, good practical control can be secured over most diseases affecting citrus trees, but sometimes losses occur despite preventive treatments applied to control specific troubles., Failure to secure adequate control lies often in a grower's lack of knowledge of conditions governing infection and the correct timing of spray applications or other treatments. In this article by F. L. Bailey, Horticultural Instructor, Department of Agriculture, Tauranga,. descriptions of the causes of . two common diseases of citrus, verrucosis and bark blotch,, are given, together with recommended control measures.

CITRUS trees generally are subject to the attacks of many diseases, which may be insect, fungous, bacterial, virus, physiological, or deficiency . troubles. Of these the insect and • fungous groups are responsible for most of the damage caused. Control of these troubles, may -be secured by the use of chemicals applied as sprays to citrus trees, or disinfectants to the wood or bark, and by attending to general orchard hygiene to reduce further infection.' The diseases with which this article deals belong to the fungous group; they are common diseases probably experienced at one time or another by all citrus growers. The losses incurred can be costly, particularly to growers new to citrus growing, if a fundamental knowledge of their cause and control is lacking. . • Citrus Verrucosis Citrus verrucosis (Elsinoe fawcetti) is a fungous disease which mainly affects lemons and is one of the major causes of fruit rejection at citrus packing houses. It is not often troublesome on other kinds of citrus grown commercially in New Zealand. The fruits, and ■ less frequently the leaves, of standard lemon varieties are affected; the Meyer lemon is subject to serious attack on both fruit and leaves. When worked on Citronelle (rough lemon) stock the -Meyer variety appears to be particularly susceptible to this disease. Infection on the fruit first appears as small, raised, scabby pimples, which are yellowish, fawn, or light brown and about the size of a pinhead. As the young fruit grows the pimples enlarge and sometimes run together to form large, irregular warts which are unsightly and hard and rough to the touch. On the leaves the symptoms may appear either as small cones up to Jin. high with a yellowish scab on the tips or as rough, fawn, flattened areas on the surfaces of the leaves similar in appearance to coarse sandpaper. Infection is seldom noticeable on old wood, but on young shoots or . laterals small, slightly raised, elongate, scabbed areas on the bark, of the same colour as those on the fruit and foliage, usually signifyinfection. Conditions Governing Infection From scientific- investigation 'it appears that verrucosis hibernates in infected leaves, and from old leaf infections the disease is spread to newly set fruit and developing foliage. The fruit and leaves become progressively resistant with age and practically immune when the fruit reaches about lin. in diameter and the leaves about lin. wide. Mature fruit and

foliage are. immune, but if infection occurs in their early stages of growth,, they can carry the fungus over, thus providing a source of infection for the next set of fruit and developing leaves, and shoots. The degree of infection seems to depend on humidity and temperature; it has been observed that infection is far more severe in citrus growing districts subject to humid atmospheric conditions when fruit and foliage are in their early stages of growth. Unlessmoisture is plentiful at the critical stage, infection does not occur. Fine weather during the flush of growth when fruit is setting and developing; does not necessarily eliminate the chances of verrucosis infection, as heavy dews, which are not uncommon in spring in various parts of New Zealand, provide sufficient moisture to enable the fungus to develop. - ’ Control Verrucosis can be readily controlled, by spraying with Bordeaux mixture at 3 : 4: 50 strength, provided applications are correctly timed. As the most susceptible period of infection is in thevery early stages of growth, protection must be given during those critical months to produce diseasefree fruit. The blossoming period for lemons is spread over a considerable time, the main spring blossoming beginning toward the latter part of October; this time and the next 2 months are the most dangerous period for verrucosis infection. In orchards or districts where verrucosis is known to be severe, the first Bordeaux application should be made in early October. This application is called a pre-blossom spray and is more in the nature of an additional precautionary measure. During November and December fruit is setting and new growth is emerging, and to keep the newly formed fruit and growth free from infection Bordeaux applications should be made about late November and in early January. The latter applications may be applied to other citrus varieties as a precautionary measure and to control other fungousdiseases to which they are susceptible. On standard lemons fruit set in autumn appears less subject to verrucosis than that set during spring. Nodoubt the weather in spring influences the possibility of infection, but Bordeaux application in May will provide the necessary protection against infection. Sometimes through change of orchard ownership -or' for other reasons spraying has been neglected and a severe infection of verrucosis has occurred. A grower faced with the problem of ridding his orchard of

the infection should adopt the following procedure: — 1. Remove from the trees all (mature and immature) infected fruits and bury them. 2. Cut but diseased and dead wood and burn it. If necessary, lightly prune the trees to facilitate spraying operations and to ensure thorough spray coverage. 3. Maintain a strict spray programme when the preliminary v operations have been completed. Verrucosis infection does not affect the qualities of the fruit, juice, and pulp, but diseased fruits are not acceptable for marketing and are a loss to the commercial grower. The disease is not difficult to control and in citrus districts where verrucosis is troublesome the cost of correctly timed Bordeaux applications will be found infinitesimal compared with the loss that occurs when infection becomesrampant. Bark Blotch

Apart from diseases causing loss of crop there are several bark .diseases of citrus which cause structural damage and even death of trees if remedial action is neglected. Of the bark diseases one of the most serious is bark blotch (Ascochyta corticoid). TT . , , . . , a * i. , Unfortunately, early stages of bark blotch infection are inconspicuous and often growers are unaware of its presence on the trees before these show some yellowing of the foliage and general unthriftiness. Bark blotch occurs most frequently and seriously

on lemon trees, but other kinds of citrus grown in New Zealand are affected. Generally, infection starts above the bud union on the lower parts of the trunk and main arms, sometimes travelling as high as 4ft. or sft. up the leaders. In the early stages infection may be recognised by light to dark brown patches of dead bark having slightly raised margins. Occasionally gumming is present around the infected areas. Usually infection spreads most rapidly upward, but slower lateral extension may encircle the trunk or main arms, causing a reduction in the sap flow, which in turn results in a change of colour of the foliage from a normal healthy green to a sickly yellow. In the later stages of infection the dead bark cracks and falls away, exposing bare wood. The fungus probably gains entry into host trees through injury such as that caused by citrus borer or : ' through mechanical injuries caused during cultivation and harvesting.' Some growers believe that only old trees are subject to infection, but that is a wrong impression, as newly planted trees have been known to ■ succumb to the trouble. Conditions Favouring Spread The spread of the disease is fostered by damp, shady conditions, often brought about by tall weed growth around the trunks or by mulching material piled up against them. Under such conditions the bark softens, is unable to dry out properly, and is in a condition suitable for the disease to thrive. In new plantings fairly serious infection has been known to occur in ■ young trees planted in areas where they have been damaged by frosts. Often cover, crops are planted in young citrus orchards to build up soil fertility. A cultivated square or circle

is kept around each tree during winter and the cover crop between the trees is allowed to grow for turning under early in spring. A “pocket” is created around each young tree, and in localities where frosts may occur the frost lodges in these pockets. When of sufficient intensity it causes damage by splitting the bark of the. tree trunks, resulting in wounds through which the bark blotch fungus can enter. Early Treatment Essential Where bark blotch appears in an orchard a careful inspection of all trees should be made and appropriate treatment carried out on infected trees as soon as possible. Preventive measures such as periodical hoeing to stop weeds from growing up around the trunks of trees should not be neglected. Care should be taken that all bulky organic material used for mulching is placed about Ift. clear of tree butts to allow air to circulate freely around them.

Spraying Programmes COMPLETE spray schedules for i| the control of citrus fruit diseases and pests in New Zealand are included in Department of Agriculture Bulletin No. 20, "Recognition i and Control of Citrus Pests and Diseases in New Zealand", which is available free from offices of the Department of Agriculture.

Lesions discovered before they develop far may be treated successfully and the tree saved. Where difficulty is experienced in finding the extent of infection, dirt or moss adhering to the trunk or limbs may be removed by using a stiff brush, which should be dipped frequently in a disinfectant solution to prevent the spread of infection to sound tissue. When the deposit has been removed the infected area will show up, usually as an irregular dark brown patch surrounded by healthy greenish tissue. The infected area should then be painted with a recommended disinfectant fluid, which penetrates the diseased tissues, destroying the fungus. Horticultural Instructors will advise orchardists on the most suitable disinfectant fluid to use. Fluid should be applied only to the infected areas and the bark immediately surrounding them, not painted over the whole of the trunk and lower portions of the tree. New bark tissue soon develops under at least part of the previously affected bark. With bad infections several applications of disinfectant at monthly intervals may be necessary to secure full control. Some time after the disinfectant has been -applied longitudinal cracking occurs followed by a general loosening of the bark covering the treated area. At that stage the flaked bark may be removed and burnt and the wound treated with a wound dressing compound, preferably a bituminous paint, which should be renewed from time to time to give protection until —:—, -ft :

exposed wood is completely covered with new bark. Depreciation and loss of trees through bark diseases are a major problem to citrus growers. Early diagnosis and treatment are essenal good yields of quality fruit to % e maintained. That can be achieved only by careful and intelligent management and by keeping trees free from structural damage s ■ Reference * “Fungous Diseases of Fruit Trees”, by G. H. Cunningham.

Holding Power of Staples in Creosoted Posts

THE opinion of some farmers and other rural workers that creosoting of posts weakens the holding power of staples is supported by the results of tests described in an article by H. R. Orman, Timber Physics Officer, Forest Research Institute, in “Forest Products Research Notes” of October 1952. Creosote was found to reduce the holding efficiency of shear staples in treated posts by 25 per cent., but this loss could be effectively overcome by increasing the shear staple size by gin. Tests were carried out on untreated totara (Podocarpus totara), creosoted larch (Larix decidua), creosoted Douglas fir (Pseudotsuga taxifolia), and creosoted Corsican pine (Pinus 1 laricio). ' The creosote loadings in larch and Douglas fir were above average, but it is not considered that the results are materially affected by higher absorptions. Staples tested for holding power were lin. barb, lin. shear, ljin. shear, and 2in. shear. There are two principal ways in which a staple can be loaded when under test: (i) ' Statically, when the -load is applied gradually and steadily; (ii) Under impact, when the load is applied very suddenly. Staple efficiency .is considered to be mainly static in practice, for although

it is probable that some of the forces tending to cause withdrawal of the staples in service may be applied suddenly or with impact, these forces will usually be applied to the ■’> fencing wires, with the result that the indirect forces at the staple, because of wire give and stretch and movement through the 'staple, will not be true impact. ' ■ ■ ' Static holding power or withdrawal resistance was therefore' taken as the best indication of staple efficiency. Results of Tests (a) Of the three species Corsican pine, larch, and Douglas fir, the last two, in general, hold staples better than Corsican pine. Untreated larch and Douglas fir are similar or only slightly superior to untreated totara. (b) Creosote affects shear staples to the same extent (a reduction of 25 per cent, in holding power) independent of staple size. (c) Increasing the shear staple in size causes a marked increase in staple efficiency for all untreated wood; the increase is also apparent in treated wood, although the increases are not quite so marked. However, this feature can be taken advantage of by using, for instance, ljin. staples in place of lin. staples in treated wood. Increasing the staple by Jin. effectively overcomes the loss of 25 per cent, in staple

efficiency in creosoted wood; use of l-gin. staples in place of lin. staples in treated timber causes an actual average increase of 27 per cent, over the figure for lin. staples in untreated posts. (d) A notable anomaly is the very low value for barbed lin. staples compared with the shear type of staples. In general, lin. shear staples have an average increase of 61 per cent, over lin. barb staples in untreated timber; in treated timber the average increase of shear over barb is only 13 per cent. In untreated larch the lin. barb staple has only 55 per cent, of the holding power of lin. shear. In treated larch the holding power of the lin. barb is increased by 20 per cent, (in Douglas fir and Corsican pine there is no significant increase), but although the holding power of the shear is decreased by 24 per cent, in the treated post, it is still- 14 per cent, greater than that of a barbed staple in similarly treated wood. The holding power of the barbed staple is considerably reduced because of the ruptured fibres that surround the staple shank. The barbed staple is apparently more effective when it is used in creosoted timber because the. barb does not rupture the wood fibres to the, same extent due to the oily nature of the creosote.

Free Bulletins for the Citrus Orchardist Nos. 20 Recognition and Control of Citrus Diseases in New Zealand. 96 Lemon Culture. 206 How to Establish a Citrus • Orchard. 241 Culture of New Zealand ' Grapefruit, Oranges and Mandarins. 285'Grafting of Fruit Trees. 303 The Propagation of Citrus Trees. The above bulletins, part of a series of over 350 on all aspects of farming, are available post free from the nearest office of the Department of Agriculture, or from the Head Office of the Department, Box 2298, Wellington.