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LESS COMMON DISEASES

The red spider is a well-known garden pest. It is not an insect, but an arachnid, Tetranychus telarius, and is a greater menace to smaller plants than to roses. However, the extent of the damage done by red spider has increased enormously since we have been using the gamma isomer of BHC and DDT, because these sprays kill, as well as pests, the natural enemies of many pests, including those that destroy red spiders, but they have no effect on the red spiders themselves. Conse­quently this pest is allowed to multiply rapidly and unchecked. Until the introduction of organic phosphates we had no spray that would kill red spiders. These sprays would eradicate this pest when applied twice at an interval of about ten days, even though it is difficult to get sprays onto the spiders, because of their habit of sheltering. But organic phosphates are not avail­able any longer to most people. In their stead we may use a new group of preparations containing Chlorparacide; these are dis­cussed in detail later in this chapter. Red spiders are rare in gardens that are watered once or more each day. They prefer dry, sheltered positions, and feed in large numbers on the under-surface of leaves, which they cover with a fine web. Red spider and mildew are the chief diseases affecting roses grown in glasshouses. Karathane will control both.

The Rutherglen bug or fly (Nysius vinitor) rarely comes in damaging numbers. Indigenous to Australia, this insect is greyish brown and a quarter of an inch long. It is very active and drops to the ground immediately a plant is touched. A beak, or rostrum, enables the fly to suck sap, causing plants to wither and die. The eggs are laid under the soil, or among rubbish and weeds. It is best controlled by E605, Systox, Lin-dane, or DDT.

Harlequin bugs (Dindymus versicolor) are well-known red orange, yellow, and black garden pests; they are sometimes called soldier bugs. They feed by sucking, and take shelter under garden rubbish, in undergrowth, and between fence-rails and pailings. Kill as many as can be gathered by hand, and use a blowlamp, or boiling water from a kettle, along the back of fence-rails.

Sunburn causes serious losses in some areas during hot, dry summer months. Sometimes it kills, but, more commonly, in lesser degree, it appears as dead branches, or brown dead patches on the sunny side of branches. It is due to the sap concentration becoming too high, secondary to insufficient soil moisture or unduly fast loss of water from the plant by evapora­tion. Trunks of standards, especially those on R. multiflora stock, must be shaded by stakes, and badly affected branches should be cut away. Sunburn must not be confused with rose-leaf scorch, which is a distinct disease, and has never been reported in Australia.

Rose-root rot can be due to any of several fungi, the com­monest of which is Armillaria mellea (honey agaric). It spreads through the soil and invades roots, growing up under the bark. It seldom extends much above ground level, but grows freely at the base of the plant. Death ensues in almost all plants attacked; sometimes the process is rapid, sometimes slow. The fungus is native to many countries, including Aus­tralia. It is common in forest soils, especially when wet or poorly drained. Dead wood, and decaying roots and bark, are ideal breeding grounds. If it is desired to replant an infected area it should be dug over several times, all dead wood removed from it, and a light dressing of lime applied. The soil should be aerated well by further digging, and, each time, it should be left in as rough a state as possible.

Stem canker is due to the fungus Conionthyrium fuckelii, which infects plants through wounds. Purplish discoloration first appears round the wound, then the bark cracks and, later, tissue-growths protrude, bearing the fruiting bodies of the fungus. The disease can be controlled by cutting away the affected parts.

Crown gall is caused by the presence in the soil of Bacterium tumefaciens, introduced by infected plants. The characteristic symptoms are the production of gall-like knobs round the crown or base of the plant; occasionally it occurs higher on the stem. The galls vary greatly in size, and cause stunting of the plant. This disease is prevalent in roses grown in glass­houses overseas, and is common out of doors in Australia, especially in alkaline soils. Affected plants must be dug up and burnt. At least two barrow-loads of soil should be removed, and there should be no replanting in that position for at least two years.