If a bulb is sickly and you are satisfied that it is not just yellowing naturally prior to going to rest and that the cause is not an obviously unsuitable soil or situation and if you cannot find, even with the aid of a hand lens, insects ( interpreting the word in a broad sense to include other small animal life as well as true insects), then suspect disease.
Diseases are usually more difficult for the amateur to identify than are insects. But be it disease, insect, or other unknown factor, if you cannot surely determine what is causing your bulbs to be sickly dig up a fair sample including roots, pack it carefully, and send it to the plant pathologist at your State Agricultural Experiment Station with a covering letter giving the name of the bulb, the culture it has received, and your observations of the symptoms which you believe indicate ill health. Ask for diagnosis and advice. You will receive a courteous reply and there will be no charge for the service.
When a disease or insect pest that requires more than simple control measures strikes, the home gardener will be wise to dig up the affected bulbs and burn them. Elaborate controls requiring special equipment are best left to the commercial bulb raiser who has a large investment in his stock. Good sanitation ( the prompt picking and burning of dead leaves, the removal of weeds and trash in the vicinity of the garden, the early control of easy-to-handle pests, etc. ) and the prompt elimination of affected individuals that are not likely to respond to treatment will go far toward keeping bulb plantings vigorous and thrifty.
Having determined what is wrong with sickly or damaged bulbs take vigorous measures to bring them back to health or destroy them. The commonest causes of trouble are here discussed.
Insects and other small creatures
Aphids or plant lice are soft-bodied slow-moving insects, green, brown, or whitish or blackish, with more or less pear-shaped bodies, slender legs, and with or without wings. They are easily visible and congregate on the young shoots and stems, on the undersides of leaves, and on the bulbs. They suck the plant's juices, cause debility and often distortion, and transmit virus diseases. They are easily eliminated by contact sprays and dusts and, in the greenhouse, by nicotine fumigation. Nicotine, rotenone, and pyrethrum used as sprays or dusts are effective.
Thrips are minute active insects barely visible without a hand lens. They are slender, short legged, and move with a peculiar weaving motion. They obtain plant juices by scraping the surfaces of leaves, petals, and other tissues with their mouth parts. This activity usually results in a streaky silvery, grey, brown, or other discoloration. Thrips hide beneath bulb scales, in flower sheaths and other out-of-the-way places. Search for them assiduously. They are especially likely to occur on amaryllises (Hippeastrum), gladioluses, hymenocallises, gloxinias, lilies, and dahlias. Effective control measures are repeated spraying with a nicotine spray.
Mites are not true insects, a technicality that depends in part upon the fact that they have eight rather than six legs. They are white, yellowish, or reddish. Some are too small to be seen by the naked eye; others are just visible. The bulb mite, which is large enough to be seen, infests the bulbs of amaryllises, crocuses, eucharises, freesias, gladioluses, hyacinths, lilies, narcissuses, tulips, and other plants. It is most abundant on rotted bulbs and it spreads the rot infection.
Recommended procedures are to destroy seriously infested bulbs and to treat suspected ones by immersing them when dormant for ten minutes in a nicotine sulphate solution ( two teaspoons of Black Leaf 40 to a gallon of water) maintained at 122 degrees F., or by dusting them with a 2 per cent nicotine dust and keeping them in tightly closed paper bags for a few days. Some of the newer miticides ( Aramite, Azo Fume, Dimite, Dow Metacide, Hexatox, Ova-tram, and others) are probably effective. Cyclamen mites and broad mites attack cyclamens and sometimes other bulbs, causing puckering of the foliage and distortion of the flowers. They are too minute to be seen by the unaided eye and are nearly transparent, glassy white, or pale green.
Dusting with Azo Fume, Dimite, and probably with some other modern miticides is effective. Badly infested plants should be destroyed. Red spider mites congregate chiefly on the undersides of leaves and can be seen with just the naked eye as they move and scurry along the slightly mealy cobwebs they spin. They are yellowish or reddish and cause the leaves upon which they feed to become yellowish or reddish in a peculiar "peppered" pattern. Red spider mites propagate rapidly when the atmosphere is hot and dry and where air circulation is poor.
Modern miticides destroy them, spraying with nicotine insecticides eliminates them, and frequent spraying with clear water is a great deterrent, but the effectiveness of all of these depends upon applying them with sufficient force to break through the webby mats under which these pests hide and upon doing it with sufficient thoroughness to wet all the undersurfaces as well as the upper surfaces of the leaves.
Mealybugs are easily visible, slow-moving insects of oval shape that are covered with a white mealy or cottony covering. They tend to live in colonies and are most likely to be found on amaryllises, urgineas, calla-lilies, nerines, bulbs usually grown indoors; but they may also infest the bulbs of gladioluses and some other outdoor kinds. They are given to hiding among the bases of the leaves near the neck of the bulb and on the bulbs themselves, often concealed under loose skin.
They cause distortion and discoloration of the foliage, weak leaf and shoot growth, and shriveling of the bulbs. The best treatment is to remove the bugs by sponging the foliage, brushing out the leaf bases and cleaning all loose scales from the bulbs. Dip the sponges and brushes in a nicotine insecticide and in severe cases soak the bulbs for two hours in water containing Black Leaf 40 at the rate of two teaspoons to the gallon, held at 100 degrees F.
Bulb flies give rise to maggots, those of the lesser bulb fly being greyish or yellowish, wrinkled, to half an inch long, and many together in each bulb . . . those of the narcissus bulb fly being white or yellowish, fat, wrinkled, and from half to three quarters of an inch long, and usually only one or two to a bulb. The grubs gnaw out the centres of narcissuses, amaryllises, galtonias, lilies, tulips, hyacinths, and some other kinds. Seriously excavated bulbs are lighter and softer than normal ones; they should be promptly destroyed.
Commercial control is achieved by submerging the bulbs for two to four hours ( depending upon their size) in water maintained at a constant temperature of between 110 and 111.5 degrees F.
Nematodes or eelworms are microscopic wormlike creatures that invade the roots, bulbs, and leaves of many species, causing foliage to be distorted and streaked and growth to be puny. Prompt destruction of affected bulbs and refraining from planting new bulbs in infested soil for two or three years or until it has been sterilized are the most practical controls for amateurs to exercise. Commercial growers use hot water immersion control as they do for bulb flies.
Soil may be sterilized by using chloropicrin, formaldehyde, or by baking or steaming.
Leaf hoppers, wedge-shaped insects that are easily seeable and that jump when disturbed, affect dahlias especially, causing them to "burn" in triangular spots at the ends of each leaflet and the leaf margins to roll inward and become brown. The plants are seriously stunted as a result and may not bloom.
Stem borers are grubs that tunnel in dahlias, gladioluses, and lilies causing considerable damage. By the time serious harm is done it is too late to save the stem. Keep a sharp watch for telltale frass ( "sawdust") protruding from holes in the stems and kill the borer inside by running a wire through it or by carefully slitting the stem and knifing it. Eliminate thick-stemmed weeds such as burdock from the vicinity of the garden. Burn all old stalks of dahlias and other thick-stemmed plants in fall.
Ants do not eat plant parts but they do carry aphids, mealybugs, and scale insects from place to place and sometimes they seriously disturb bulbs by tunnelling among their roots. Chlordane used as recommended by the manufacturer and various commercial ant baits destroy these creatures.
Scale insects occasionally occur on the stems and foliage of bulbs including cannas, caladiums, hymenocallises, and gladioluses. They appear as small grey, brown, or black "bumps" that do not move. Repeated spraying with nicotine insecticides or with Melathion brings them under control.
Slugs and snails work at night eating shoots and leaves. They leave yellowish, silvery, or sometimes nearly colourless shiny trails where they have been. They hide during the day in dark, dank places, under leaves, planks, loose bricks, in loose stone walls, in the bottoms of hedges, etc. Clean out their hiding places. Use one of the commercial slug baits or trap them by leaving loose shingles on the ground each with a piece of lettuce beneath. Each morning they may be collected from under the shingles and destroyed by dropping them in a strong solution of salt and water.
The tarnished plant bug is a flattish, oval, quarter-inch-long creature of brownish "tarnished" appearance with a yellow triangle with a black dot on the lower part of each side. It causes the buds of dahlias, gladioluses, and other flowers to die or open imperfectly.
Control is difficult. Remove and burn all trash in the fall. Keep weeds down. Dust repeatedly with tobacco dust, or spray repeatedly with a nicotine insecticide.
Caterpillars of various types eat the foliage of dahlias, cannas, hymenocallises, amaryllises, and some other bulbs. Dusting with lead arsenate or calcium arsenate at the first evidence of damage is effective.
Cutworms of surface types are sizeable, fat, slow-moving caterpillars, greyish, brownish, or greenish, that live in the soil and nip off the shoots of newly planted dahlias and other young plants at about ground level. They feed at night and remain coiled up just beneath the ground surface by day. Tar paper collars made to surround young shoots are on the market and are effective preventatives. Poison baits put out in the evening according to manufacturers' directions are also good.
Wireworms are smooth, brown, slow-moving, hard-skinned creatures, wormlike and from half an inch to one and a half inches long that live in the soil and feed on bulbs of dahlias, gladioluses, and some other plants. They are usually most prevalent in land that has recently grown grass. Wireworms may be trapped by planting half potatoes cut side downwards two or three inches in the soil and spaced about a yard apart. Take the potatoes up after about a week and destroy the wireworms adhering to them.
Millepedes are many-legged brown worms up to an inch and three quarters long that wrap themselves in tight watch-spring coils. They are found in rotting bulbs but apparently they eat only rotted parts and do not harm sound bulbs although occasionally they damage seedlings and young roots. They may be baited by spreading Paris green on cut potatoes and placing these under boards ( to prevent birds from being poisoned ), or by soaking the soil with a 1:1000 solution of deadly poisonous bichloride of mercury.
Such rodents as squirrels, chipmunks, and mice that burrow or dig may cause a good deal of disturbance to bulbs, and mice even eat certain kinds including tulips. Moles rarely if ever eat bulbs but they disturb them and make runs along which mice travel in search of tasty morsels, which are very likely to be your choicest bulbs.
There are no easy ways of circumventing these creatures. Trapping, poisoning, and shooting ( where these measures are lawful) help. The presence of an ambitious, not too well-fed cat around the garden has a discouraging effect. In extreme cases bulbs may be planted in baskets of fine-mesh chicken wire to prevent mice from getting them, or entire beds may be protected by enclosing them in a foot-wide strip of fine wire mesh sunk vertically along the bed margins in such a way that about two inches protrude above ground and ten inches extend below.
Rabbits eat the foliage of crocuses and some other bulbs. Fencing these creatures out of the garden is the best insurance against damage. Shooting or trapping, where permissible, are alternatives. Friend cat may have an inhibiting effect.
Birds sometimes destroy the buds of crocuses and other small spring-flowering bulbs. It is said that they do this when water is not readily available. The answer seems to be to provide them with drinking water, and it might help too to make sure that they are fed.
Virus diseases, several of which are serious, affect bulbs, notably lilies, amaryllises, narcissuses, tulips, and dahlias and in addition irises, calla-lilies, ornithogalums, galtonias, hyacinths, gladioluses, and lachenalias as well as others. Most are characterized by a loss of a green colour in the foliage in a mottled or ring-spot pattern and by a stunting of growth and sometimes other distortions.
Affected plants never recover. All vegetative propagations from them are infected and they act as sources of infection because these diseases are transmitted by insects puncturing a healthy plant after an infected one and by knives and other contacts that permit the sap of a diseased plant to enter that of a healthy one. Since there are no cures the control of viruses depends upon planting healthy stock, promptly removing and burning infected bulbs, keeping down thrips, aphids, and other insects that transmit these diseases and eliminating weeds that may be sources of infection.
Botrytises are a group of fungi that cause "tulip fire" and "narcissus fire" as well as "grey mould blight" on amaryllises, dahlias, gladioluses, lilies, eucharises, snowdrops, and other bulbs and the "leaf and stalk blight" of jack-in-the-pulpits. Botrytis diseases are most prevalent in wet weather and where air circulationis poor.
Tulip fire manifests itself by the presence of malformed leaves and shoots and by the presence on the leaves of large light withered patches ( do not mistake frost injury for these) upon which a grey moldly growth develops. Leaves and petals are marked with small white or brown spots. Control consists of picking off and destroying occasional leaves or petals that show infection and of removing seriously affected bulbs promptly and in such a manner that they are not shaken to disperse spores onto other plants. Spraying with Fermate is helpful.
Narcissus fire causes spotting and rotting of the flowers and foliage. It is reported from the Pacific Coast only. Remove infected flowers and leaves. Spray early in the season with Fermate. Grey mould blight is best combated by sanitary measures, the prompt picking of all faded flowers and rotted or blighted leaves. Drop these as collected into a paper bag and burn them. If the rot affects the base of the stem or the bulb dig it up and destroy it. Provide good ventilation and appropriate temperatures in the greenhouse and avoid wetting foliage except on bright days when it will soon dry.
Other moulds, rots, and blights may infect both the underground and aboveground parts of bulbs. These diseases are caused by fungi, bacteria, and other organisms. They are often most severe where soil or atmospheric conditions are too humid and too warm. Exposure of dormant bulbs to light and good ventilation and dusting them with sulphur or soaking them in Semesan may bring about control.
Spraying or dusting foliage, at the first signs of infection, with Fermate or with Bordeaux mixture is often effective. In some cases, "gladiolus scab" for instance, soaking the affected bulbs for two hours in a 1:1000 solution of mercuric chloride ( very poisonous) produces results. "Leaf spot" and "leaf blotch" diseases occur on calla-lilies, begonias, gladioluses, irises, eremuruses, and other bulbs. Symptoms consist of various foliage spottings and blotchings. Controls are spraying with Bordeaux mixture or Fermate, keeping the foliage dry, and picking and burning affected parts promptly.