RUST AND APHIS

Dominion, Volume 11, Issue 66, 11 December 1917, Page 8

RUST AND APHIS

TROUBLE IN THE SOUTH ISLAND,

Tho prevalence of rust in tho. Ellestoere and) Eouthhridgo districts recently was responsible for an inspection being made by Dr. A. H. Cockayne-, Government Biologist, of tho affected area (says tho Christchurch "Siui"). His report has been forwarded by tho Director of the Fields Division (if tho Agricultural Department to Mr. A. MacPhersoi), Fields Supervisor of the Bouth Island. Whilst it particularly concerns the districts'mentioned, sitr. nificauce can be attached to the fact that rust and aphis are fairly general in many portions of tho Canterbury wheat belt,.

Dr. Cockayne states:— , "In .the first place, it may bo witfl to ..outline the conditions that have ocmrred since sowing time. The rainfall durng the early period of growth was low, but extremely well distributed. As ■the water content was low in the lower levels of the soil, in conscquence of the two previous. seasons' drought, tho cereal crops became extremely surfacerooted, due to tho small falls of rain which occurred fairly regularly. Since the spring the rainfall has been small, and, m fact, at Southbridge the piesent twelve months' rainfall is less than the preceding twelve months. Those conditions appear to have favoured an excessive development of corn aphis (aphis graminis). This insect is always present in fair numbers, but is kept 111 check' by the "seven spot ladybird. This year it seems that the ladybird beetle must have been later than usual in appearing in sufficient numbers to keep the aphis in check, and, in consequence, this has increased enormously, and sapped the vitality of many of the cereal crops. During my visit it was notice,'ahle that in crops that had been very badly infested, but were now more or less free, ladybirds were present in large numbers. -

" "In crops where tbe aphis was-still abundant, very few ladybirds could be ' seen. Tho presence of aphis in large quantities, even if tbe beetle were comparatively absent, would not have been so particularly serious had the cereal plants been deeply rooted, and not so surface-rooted as they are. Tbe result has been that as soon as the plants lost vigour through tho drying of the upper surface of the soil, the toll levied by the aphis was too severe for their proper development. Again, many of the crops (and this refers especially to Dun oats) have become very badly infested with rust; in fact, never before in my experience, except with regard to white«oats in certain parts of the North Island, have I seen such badly rusted crops -while yet in the flag state. As far as I could see, "the main damage was caused by corn aphis, against which .there aro no practical methods of control, nor are any methods of control necessary in normal years when the ladybirds are present in their nsnal numbers. _ The actual damage caused by aphis is greatly intensified by the surface-rooted development of many of the crops and their inability to secure sufficient water for their increased requirements, due to the drain 011 them through aphis attacks.

"The extreme prevalence of rust may or may not liava been induced by the condition caused liy aphis, but it is a significant fact that crops badly affected by aphis are now .falling ready victims to rust. So far as future treatment to avoid a recurrence of rust is concerned, there is little of a practical nature that can be suggested. I consider that on many soils some system of green manuring should be adopted, as this should have a great effect on the better conservation" and distribution of the soil moisture."