31

H.—34.

Potato-virus Diseases. The research included—(a) A survey of potato crops to determine characteristic insects. This phase is now completed, and the following characteristic insects have been found : (1) Macrosiphum gei ; (2) Myzus persicae; (3) Myzus pseudosolani ; (4) Melanophthalma gibbosa ; (5) Thrips tabaci (provisionally named) ; (6) Erythroneura sp. (specimens being sent to Imperial Bureau for identification) ; (7) Collembola —(a) Fam. Sminthuridae, (b) Fam. Entomobryidae (specimens in hand of Imperial Bureau for identification). The dipteron Lauxania bilineata is also fairly common on the foliage. The aphid predators Coccinella 11-punctatae and Melanostoma fasciatum are present wherever the plant-lice are sufficiently abundant. (b) Origin of seed trials, involving a survey of selected areas for potato-growing to determine relative abundance of characteristic insect fauna. The selected areas are situated in scattered localities in the South Island, and during the season it was necessary to visit these localities on three distinct occasions. The seed-potato trials will extend over a period of four years, and the results of the entomological survey are being forwarded to the Crop Experimentalist for proper recording along with other data. (c) Under laboratory conditions rearing and determining which of the characteristic fauna is a virus vector. Everything is now in readiness for the prosecution of this work, and it will be com menced as soon as the mycologist section can supply us with the so-called virus-free potatoes. It is proposed to experiment with insects 1, 2, 3, 5, and 6, named under section (a). Dry-rot in Swedes. Laboratory experiments are in progress to determine whether the normal spread of dry-rot in a crop can be attributed to insects. Two insects are being used for this work : (1) A beetle of the family Staphylindae. These insects are present in comparatively large numbers on infested swedes. (2) A dipteron which breeds freely in the rotting bulbs. It is as yet too early to give any definite results. Aphis (Brevicoryne brassicae) on Swedes. Opportunity was taken during the " origin of seed trial " inspection to observe the incidence of the aphides on swede crops in the South. Distribution was general over all of the swede-growing areas. Several parasites of Brevicoryne brassicae and Myzus persicae on swedes have been found, and five or six species have been collected. Notes on the abundance of B. brassicae during the season have been made. Ragwort. The stem-boring moth Homoeosoma Jarinaria is being tested under laboratory conditions for its effect on ragwort. Mycology. (1) Cereal Diseases. (а) Smuts. —Work during the year has been confined to bulk treatments by the hot-water process for elimination of loose and covered smuts of barley, wheat, and oats. An investigation was carred out to determine whether loose smut was more prevalent in plants grown from apparently healthy seed taken from smutted plants than from those developed from seed taken from contiguous, non-smutted plants. Results showed no appreciable differences in percentages of infection between the two lines. Several new seed disinfectants were tried out during the year, but none showed promise of beingbetter than any of those in general use. (б) Wheat-scab. —Twenty-two different seed-treatments were conducted during the year to determine whether this disease (which preliminary laboratory-work showed to be seed-carried) could be controlled. All failed owing to secondary infections occurring in the plots. (c) Stripe Disease. —Forty-eight experiments on control of this disease were conducted on the farm. Although several of these treatments prevented seedling infection, all plots became infected subsequently from air-borne spores. Further studies in dissemination are therefore necessary before attempts at control on a field scale become possible. (d) Black End. —Experiments to determine the cause of this condition—whether it was seedcarried —and a possible control by seed-treatment, were carried out at the farm and at Ashburton. The cause is still under investigation, pathogenicity studies being under way in the laboratory. The disease has been proved to be carried with seed ; control experiments failed in that only partial success was met with, owing to only a few possible treatments being tried. (e) Take-all. —Attempts to infect an area on the farm with the disease again failed. This is the third consecutive time that failure has resulted. Consequently a comprehensive laboratory study of the organism is considered necessary before further field-work can be undertaken. (/) Rusts. —Preliminary field studies have been commenced with a view to determining whether any races of oats grown in the Dominion show promising indications of being resistant to physiologic forms of black-rust and crown-rust, thirty-two plots being sown on the farm. Certain of these show varying degrees of resistance, but little work can be carried out until the rusts have been studied from a physiological form viewpoint. Summary. —The present position with regard to cereal-diseases is that smut-elimination on a field scale has proved practicable. With all other diseases under investigation, it has been found that a detailed study of each under laboratory conditions is necessary before field experiments on control can be undertaken with any degree of confidence.