ORCHARD LOSSES

New Zealand Herald, Volume LXXI, Issue 21893, 31 August 1934, Page 5

ORCHARD LOSSES

STONE FRUIT DISEASES CONTROL OF BROWN ROT CAWTHRON INSTITUTE BULLETIN BY K. M. CURTIS, M.A.. D.I.C. D.SO. Although brown rot of stone fruit has been prevalent for many years and the life history of the fungus causing it long known in essential details, the control of brown rot on the practical scale is still by no meanG a certainty for the average orchardist in districts of heavy rainfall or of high atmospherio humidity in spring and summer. Nevertheless with attention to the phases that need this care, brown rot can be kept tinder reasonable control. The fungus produces throughout spring and suirmer a succession of crops of conidia (summer spores), the number of the successive crops being chiefly dependent on the distribution of the rainfall. If it is distributed over a large number of days, the number of conidial crops during a season will be high. With a small rainfall, or a heavier one limited to a few wet periods with long intervals of fine weather, the number will be considerably less. These conidial crops may arise on infected tissue in the flower, fruit, leaf, shoot or lateral. Appearance of Leaves In the leaf the infected area takes the form of a light brown patch whose surface at first is slightly moist. It soon dries to brittleness and then pot infrequently falls away, leaving th« leaf with a shot-hole effect. l<eaf infection does not regularly occur even when much brown rot is present elsewhere on the tree, but it may be widespread if heavy rain occurs while the leaves are still young, or if the leaves were previously heavily infected with leaf-curl. If a largo area of the leaf is infected with brown rot it usuallv falls from the tree. Infection in the blossom _ merits greater attention than that in any other part of the tree except the fruit, for the success or non-success of the season's crop largely depends on the prevalence of blossom infection. It mav take the early form of bud infection before the petals expand. Infected buds become noticeable by the time the uninfected blossoms are fully open, for they persist in closed condition in contrast to the expanded healthy blossom. Susceptibility of Petals Once the blossom is fully open, particularly if it is of the flat-petalled, wide-spreading type, it is extremely susceptible to infection, since brown rot spores falling from the air on the honeycoated centre of the blossom find there an ideal medium for germination and subsequent' penetration into the flower tissues Blossoms infected when open can be detected within a day or two owing to their petals turning a prematurely dull buff colour, and by the gum that exudes holding the discoloured petals attached to the flower for some little time after normal petal-fall. Within a few days of infection both buds and blossoms produce conidia on their surface, and they continue to produce conidia at intervals for the rest. of the season. Infection of the blossom leads to a triple result. If it takes place early the resulting conidia may bring about a second or even a third wave of blossom infection before the blossoming period closes for late-flowering varieties. In the second place the fungus in the blossom grows down through its pedicel into the shoot that bears it, and there as a rule forms a canker that eventually girdles the shoot. By this shoot infection the fungus has succeeded in establishing itself in the perennial tissues of the tree, and owing to the girdling any healthy fruit higher on the shoot is doomed through the cutting off of its sap supply by the dead tissues in the girdled region. Moreover, as the result of becoming established in the permanent tissues the fungus is in a position to maintain the production of successive crops of conidia on the cankered surface for the remainder of the season, and thus in its third effect blossom infection menaces all fruit in the vicinity that succeeds in attaining the susceptible pre-matur-ity stage. Introduction Through Bark

Although most shoot infection is the result of growth of the fungus from infected blossoms, insect punctures or wind abrasions may weaken the bark sufficiently to enable the fungus to penetrate directly from the surface. In most stone fruits shoot or lateral infection by the brown rot fungus results in short cankers. But in the cherry the rate of growth of the fungus is faster and a definite die-back may result. Even in a small canker, however, penetration toward the centre is usually sufficiently deep to impair the sap flow to tissues lying above. In some seasons blossom infection is hardly noticeable, but the significance of the blossom phase is fully realised throughout the world and the necessity for preventing or eliminating it is unquestioned. In New Zealand some orchardists still tend to concentrate their chief attention on protecting the fruit as it approaches the ripening period. Efforts at this time should certainly not be relaxed as this is the second susceptible stage, but if the precaution has not been taken earlier of either preventing or removing the effects of blossom infection, the orchardist is in effect allowing the fungus to become securely entrenched in masses in close proximity to the fruit that he intends only later to endeavour to protect Steps for Control In addition to tlie bud-movement and early pink sprays before blossoms open, repeated, vigilant a (id systematic inspection should be made of all stone iruit trees from the beginning of blossoming time until after the petals fall. Wherever an infected blossom or blossom-bud is detected the shoot bearing it should be immediately cut out, and all such shoots promptly collected and burnt. This may involve extra work in the first year in which it is practised, especially where infected fruit was not removed from tree and ground at the close of the previous season's picking. 13 nt in the second and succeeding springs the time required for dead blossom removal is so short that the orchardist may commit the error of concluding that as so few blossoms are infected they cannot be of great significance to the fruit. Nevertheless, however few blossoms are infected their removal should lie an act of standard routine year after year, and omission of the practice feared as greatly as omission of the pre-maturity spray when the fruit begins to ripen.