Fancy Flowers, But Disease-Prone

Press, Volume CIV, Issue 30919, 27 November 1965, Page 24

Fancy Flowers, But Disease-Prone

Irises are in flower now and gladioli are coming into flower; both these plants provide most attractive displays. Their value is their appeal not only outside but also inside as material for floral arrangements.

However, both are subject to a number of pests and diseases and it is proposed to look at some of the more prevalent ones.

One of the most widespread diseases of both irises and gladioli is virus. Iris mosaic is restricted to the bulbous irises, the Dutch, Spanish and Iris tingitana. It is spread by aphides and causes light green streaks on the leaves and flower sheaths and light, sometimes dark, streaks on the flowers. Once infected the whole plant carries the disease. There is no control other than burning if the virus has completely weakened the plant and thus severely reduced the growth of flowers. Poor cultural conditions and general neglect often result in the disease having a more adverse effect on the plants. Dusting the bulbs with lindane before replanting, and spraying the plants with malthion, lindane or parathion as soon as aphides are noticed on the leaves will help to keep healthy subjects free to a certain extent. CUCUMBER VIRUS Gladioli are susceptible to cucumber mosaic virus, which can cause severe blotching of the flowers. It is transmitted by aphides as well as in the sap. Pea mosaic virus, which causes leaf mottling and colour striping of the flowers, is also a common disease spread by aphides. Control of aphides and the destruction of infected plants are again the only means of control. Warm, wet weather is often conducive to severe infection of leaf spot on irises of all types. Small, light brown spots, more prominent towards the tips, are the symptoms to look for. These spots gradually develop grey centres and the tips of the leaves yellow and wither. As even light attacks can considerably affect plants, bulbs and flowers, immediate control is necessary. The infection becomes increasingly more difficult to control as it establishes itself. Protective sprays of bordeaux. maneb and ztaeb are advisable and old leaves should be collected and burnt. CORMS INFECTED

Botrytis can attack the corm, leaves and and occasionally the basal region of the steam, at or near ground level, of gladioli. The new corms are infected by the' disease spreading from the old one. It can be recognised by feeling the basal plate of the corm, which should feel

firm. If infected it will be spongy.

White-centred brown lesions are signs of botrytis infection on leaves. Moist conditions induce spotting of the flowers as well and this is often accompanied by a grey mould. Initial treatment with a fungicide when lifting the corms in autumn is always sound policy and should be accompanied by correct curing and drying. As there are various rots apart from botrytis which can harm the corms the use of dips containing aretan, captan, mercuric chloride, or formalin is recommended. Whichever material is applied, it should be prepared and used according to makers’ recommendations. Irises may be similarly treated as they, too, are susceptible to a number of rots. BLACK BLOTCHES Irises may also be attacked by two other fungus diseases, one of which, ink spot, can be especially troublesome among Iris reticulata. The other, iris rust, is not as

troublesome in New Zealand. Ink spot causes black blotches on the leaves in late winter and early spring and possible stunting of the whole plant thereafter. Bulbs affected may be just a shell containing a residue of black powder. Regular spraying with Bordeau mixture will give control and infected bulbs are best deposited in the incinerator.

Zineb or bordeaux at regular intervals will check rust on iris, some of which, such as Iris fulva and Louisiana hybrids, are more susceptible than others.

Bulb mites may be found on both iris and gladioli and immersion in hot water will kill them.

Another pest common to both plants is thrips, which give the leaves a silvery appearance. They can be controlled by D.D.T., malathion or lindane. In association with remedial measures, correct cultural treatment and attention will go a long way towards avoiding many of the pests and diseases mentioned.