Pest And Diseases Of Hostas

Hostas are relatively free from disease but slugs and snails can do considerable damage, though gardens vary greatly as to their populations, some being hardly affected at all. A variety of remedies is available, but experience suggests that the most effective regimen is to water a liquid metaldehyde formulation on to the hostas and the ground around them in early spring, just as the emerging shoots start to push through the ground. The timing is crucial because this is when slugs and snails emerge from hibernation. The treatment should be repeated at weekly or fortnightly intervals for six to eight weeks, by which time the liquid metaldehyde will have been applied into the unfurling and expanding leaves in which many small slugs hide by day. Early treatment is particularly effective at destroying the small dark brown or black keeled slugs that live just under the surface of the soil and do most of the early damage to soft young leaves - damage which, in hostas, is all too often visible for the rest of the season.

After that the gardener may resort to slug pellets. Most contain a chemical which actually attracts slugs, and heavy-handed use may exacerbate the problem rather than solving it. It takes very little of the chemical in the pellets to kill a slug, and therefore a lot of pellets are not needed; the art is to find where the slugs hide by day and, very sparingly, scatter the pellets there. Typically slugs like cool, damp places to hide in, and often the gutter between a border and a lawn provides the perfect place. A few pellets scattered in the gutter will be far more effective than a thick ring of pellets poured round a hosta, but they must be applied frequently as they are only viable for about 3-4 days. They are most effective applied after rain, and in damp, muggy conditions when the slugs are at their most active.

Pellets usually contain either metaldehyde or methiocarb, and the latter is generally the more effective, though some people like to alternate them. Both are exceedingly poisonous to humans, and should be treated with respect. It is also possible to obtain pellets based on aluminium sulphate, which are said to be more environmentally friendly: unfortunately they are less effective at destroying slugs and snails. Pellets are usually blue, as it is thought to be a color which birds cannot see. If there is any reason for concern about pets eating the pellets, place the pellets in the centre of short lengths of clay land drain or plastic guttering or under a piece of slate, where they cannot easily be reached.

The most effective, and cheapest, method of slug control is to go round the garden at dusk with a bucket of salt water; several hundred slugs can be scooped up in a very short time, especially after rain. Many gardeners resort to slug pubs, upturned grapefruit halves, crushed eggshells and other ancient remedies, but these do nothing to improve the beauty of the garden.

Recently a biological control for slugs has become available. A nematode, it is specific to slugs and will do no harm at all to snails. The nematode is mixed with water and applied to the ground around hostas through a watering can. The nematode then parasitizes any slugs that come in contact with it, multiplying in the slug's body and reinfecting any slugs that come to eat the dead slug.

An alternative or complementary way of controlling slugs and snails is to understand their part in the natural scheme of things, and to avoid creating in the garden or around the hostas conditions in which these pests can thrive. Their role is to clear the world of rotting vegetation. If slugs and snails did not exist, gardens would soon be swamped by decaying debris. Keeping the garden free from dying leaves and other garden rubbish such as old bricks and stones under and behind which they can hide will deter them, since such materials provide not only an ideal home for them but also a perfect breeding ground. The leaves of hostas and their companion plants should be removed as soon as they start to turn yellow or show signs of decay. Such a philosophy presents a dilemma for those who like to mulch their hostas, a practice which without doubt helps to conserve moisture and to promote healthy growth, but also tends to provide a cool, damp hiding place for slugs and snails. Regular and thorough cultivation of the borders helps to expose the eggs and the slugs to their natural predators but this precludes the use of a mulch.

The depredations of slugs and snails can be more easily be controlled in pots and containers than in the open garden. These should never be stood directly on the ground since the ground beneath them will be cool and damp, providing slugs and snails with an ideal habitat. Instead they should be raised about 4cm (1 1/2 in) above the ground on small feet, broken bricks or broken tiles. Provided this is done and the foliage of the hosta in one pot does not touch the foliage of another hosta, they will very often get through the season with no damage at all, but extra steps may need to be taken. Pellets tucked out of sight around the rim of the pot will prove effective, and are unlikely to be discovered by pets. If a band of petroleum jelly or fruit tree grease about 4cm (1 1/2 in) wide is smeared in a circle halfway between the top and bottom of the container this will greatly deter the slugs and snails from reaching the hosta.

Hostas vary greatly in their resistance to slug and snail damage. Those with thick leaves such as H. 'Sum and Substance' and H. 'Green Sheen' are generally little damaged, while hostas with thin leaves, such as H. 'Undulata' and forms of H. sieboldii, suffer very badly.

Vine weevil

It is the vine weevil grubs that do the real damage to hostas, eating the roots. The grubs or larvae are C-shaped, fat, creamy-ivory in colour, and about half the length of a thumbnail. If discovered they should be destroyed at once - they are easily squashed underfoot, and goldfish find them delicious. The adults, which are slow-moving beetles, eat the edges of leaves, leaving characteristic notches. The adults can be controlled by the use of organophosphates, but it is the destruction of the grubs that is really important. A biological control, a parasitic nematode, is now available and is highly effective. As with the slug nematode, it is applied through a watering can.

Vine weevils are usually more of a problem where hostas are grown in pots than in the open garden, though they can become a problem in boggy or ill-drained ground. They tend to infest pots which have contained the same compost for a year or more, so one way of controlling them is to repot all the hostas every year.

Diseases

Given good cultivation, hostas are mostly disease-free, but there are just a few that may occur on occasion. The first is viral infection, which usually takes the form of a yellow mosaic appearing on the leaves, the yellow following the lines of the major and minor veins, but on occasion manifesting itself as large yellow blotches on the leaves or even yellowing of almost the whole of the leaf surface. It may also make the growth of the leaves dwarf and congested. The two viruses which can affect hostas are arabis mosaic virus and tobacco rattle virus and these are usually spread by aphids or by dividing hostas with garden tools which have previously been used on infected plants. Mineral or chromosome deficiency shows up in leaves as blotches or mottling and this is sometimes mistaken for virus infection.

There is no cure for virus infection and only one recommended treatment: affected hostas should be dug up and burned as soon as possible before they infect other hostas. Apart from H. 'Crispula', H. 'Sea Sprite' and H. 'Tardiflora', the most commonly affected hostas seem to be those with glaucous blue leaves.

Another disease found in hostas is crown rot. It usually only affects hostas growing in warm climates or under glass, though over-compacted growing media may also cause it. The leaves turn yellow and fall away from the rootstock. The plants should be dug up and pulled apart, affected pieces being thrown away or burnt and only firm, healthy pieces being retained. Since crown rot is a fungal infection, these pieces should be dipped in a fungicidal solution before being potted up and grown on.

The only other disease to cause a problem to the hosta-grower is leaf spot, which is caused by the fungi Alternaria, Plyllostricta and Colletotrichum omnivorum. The treatment of leaf spot is two or three sprays at fortnightly intervals with Benomyl or Thiram.

Spots on hosta leaves can be caused by damage from misdirected chemical garden sprays and other air-borne pollutants. Pinhead-sized brown spots sometimes seen on leaves in late spring and early summer are caused by the leaf-cells bursting due to slight frosts which often occur some time after the leaves have unfurled, and this damage is not apparent immediately. Frost damage can also cause the edges to turn brown. Leaf scorch, similar to leaf spotting, is harmless though unsightly, and is caused by small drops of cold water falling on to leaves growing in strong sunlight.