H.—34.

Tests have shown, that on the variety Cox's Orange, and probably also on Delicious (though records are not yet complete), both lime-sulphur and Bordeaux can cause an appreciable amount of russet if applications are delayed a week beyond early green-tip. Foliage injury which sometimes arises following use of two or more applications of 1 per cent, summer oil appears to be a cumulative effect. A single application of 3 per cent, concentration produced almost identical injury to that caused by three sprays of 1 per cent, concentration applied at fortnightly intervals. V. Therapeutant Testing. Certification.■ —Plant Diseases Division lists of certified therapeutants have remained substantially the same during the past twelve months. A few new products have been added, and an equal number of older ones withdrawn. In a few instances manufacturers have been requested to improve the quality of their products, as these were somewhat below the standards adopted by manufacturers of others sold at comparable prices. It is gratifying to note that numerous samples taken in the field have proved on analysis to be equal to those on which the certification was originally based. None of the hydrated lime manufactured in New Zealand attains the standard required for certification. Since importation of hydrated lime is prohibited, growers are forced to use non-certified, inferior products. PEARS. Plant-protection Experiments. I. Entomological Studies. Mealy-bug.—Glasshouse experiments of the Plant Diseases Division showed that mealy-bugs taken from pears could be killed with a single application of 1 per cent, summer oil plus 0-012 per cent, rotenone. Chemically pure rotenone and an equivalent of ground derris were equally effective. Oil or rotenone used separately gave poor results, as did a nicotine-oil spray. Field trials of rotenone-oil combinations are being undertaken. 11. Spraying Experiments. Modified Spray Programmes. —At the Havclock North Orchard the Plant Diseases Division reports control of pear black-spot, Venturia pirina, with Bordeaux IJ-2-50, instead of 3-4-50, after blossoming, with marked reduction in russetting. Lime-sulphur sprays sufficiently strong to check the disease caused very little russet but seriously injured the foliage. As in previous seasons, this disease was much more difficult to control on trees which had been heavily infected in the past, indicating that the source of carry-over was by means of infected twigs on the trees. Russet and Spray Injury. —It has been found that the Winter Nclis pear has a fairly well-defined period during which it is susceptible to spray-induced russet. STONE ERUITS. Plant-protection Experiments. I. Mycological Studies. Silver-leaf. —In last annual report it was noted that at the Plant Diseases Division two hundred young peach-trees had been artificially inoculated with the organism Stereum purpureum, and that they had been subsequently injected with numerous chemical compounds. No treatment succeeded in checking the spread of the disease, despite promising results secured in earlier experiments. The work is being continued with injections made at different periods and in more mature trees. Brown-rot.—The Cawthron Institute has maintained the usual notification service regarding the date of ascospore maturity. 11. Physiological Studies. Brown-spotting of Apricots. —In co-operation with the Horticulture Division of the Department of Agriculture, the Cawthron Institute has continued observations on the trees under test at Alexandra. Confirmation has been obtained of the control of the disorder by either a soil dressing of \ lb. hydrated borax per tree or aOT per cent, spray of the same compound. Trees so treated were practically free from the ailment and suffered very much less from bud-drop and leaf malformation. Pitting of Cherries.—lt was hoped to obtain evidence in the field in support of the analyses made by the Cawthron Institute, suggesting that pitting might be a boron-deficiency ailment. Again, conditions were such that pit failed to develop even on the untreated trees. Gumming of Plums. —A field test by the Cawthron Institute on the possible control of this trouble in Angelina plums gave inconclusive results, for treated trees still showed a fair percentage of gumming. It is possible that the amount of borax (| lb. per tree) was insufficient to give full control in the large trees used in the experiment. Die-back.—A considerable number of stone-fruit trees in Central Otago showing symptoms of die-back have been examined by the Plant Diseases Division, and many have been dug out so that the roots could be studied. All attempts to isolate a pathogen failed, so the work is being continued on the assumption that the condition is physiological in origin.

7—H. 34.

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