Lily Diseases

In the earlier years of cultivation the susceptibility of Lilies to diseases limited their popularity as garden plants. However a few individuals saw potential beauty in the size and shapes of the flowers and continued to experiment and hybridize. Over time many varieties were developed that had tolerance to the more common viruses and diseases. This resistance has allowed modern cultivars to become common items in home gardens and guaranteed Lilies a secure position in the commercial cut flower and potted plant industry.

An important factor in the resistance to viruses and diseases is the over all health of the plant(s). Just like all living things a healthy vigorous plant is resistant to sickness and a organism under stress is susceptible to diseases. Things like drainage, soil texture, air circulation, light, water, food, soil pH all influence the health of your plants. Gardeners who understand the requirements of their plants eliminate those factors which effect the plants health in a negative way and increase positive health influences.

Viruses

Viruses are very simple organisms, they are nothing more than RNA contained within a protein shell. They are so small that they can only be seen with the aid of an electron microscope. They will invade a cell injecting its own RNA into the cell, hijack the cells DNA and cause it to replicate the virus RNA over and over again. The new viruses grow in the plant cells and mature. When they are mature they burst open from the cell and move on to infect other cells and start the process over again. When this happens the normal plant cell functions are interrupted and symptom usually show up on the exterior of the over all plant in the form of streaking or mottling, twisted growth, reduced plant size and/or rings on the bulbs. Lilies that are weakened by viruses are more susceptible to other forms of diseases. Many species and some cultivars are more susceptible to viruses, others tolerate them with little or no indications.

Three of the most common viruses found in lilies are often transmitted from plant to plant by aphids that have bitten into the plant and ingested the virus. When they land on a non-infected plant they pass the virus to it when they bite into it. These viruses are Lily Symptomless Virus (LSV), Tulip Breaking Virus (TBV), and Cucumber Mosaic Virus (CMV). There are a couple of lesser important viruses that do not effect lilies as often. These are Arabis Mosaic Virus (AMV), Lily Virus X (LVX), Tobacco Mosaic Virus (TMV), Tobacco Ringspot Virus (TRSV) and Tobacco Rattle Virus (TRV). AMV, TRV and TRSV are soil born and transmitted via nematodes.

Lily Symptomless Virus

This is the most common lily virus and is found in many species and hybrids. It can occur with virtually no outward visible symptoms except possibly some stunting in the growth. It does however weaken the plant leaving it more susceptible to other viruses and diseases.

Tulip Breaking Virus

For allot of gardeners who like to grow a variety of bulbs for spring colour Tulips are quite often the a popular choice. Rembrandt or painted tulips get their characteristic streaking colour pattern from the Tulip Breaking Virus hence its name. This virus can be easily transmitted from these tulips to your lilies, so for this reason tulips with broken colours should not be grown close to your lilies. It causes mottling of the foliage, lessening of the colours of  darker flowers and sometimes the breaking of the colour.

Cucumber Mosaic Virus

This virus can cause very streaky foliage colour and distorted, brittle leaves and flowers. This virus has a wide range of host plants, including many common weeds and cultivated plants. The leaves may become pale and the plant stunted. This virus is also spread by aphids so regular weeding to cut down on potential sources is very important. Aphid control is another way to reduce the chances of this virus taking hold in your collection.

To identify all the possible symptoms to identify a virus is beyond the scope of this website. I suggest you find reliable resources either on the internet or through reference books to look for pictures and descriptions of the various viruses. If you find you have a Lily that shows sign of a viral infection, dig out and destroy the entire plant(s) preferably by burning. Disinfect all your tools immediately, soaking in bleach usually works well.

The best way to reduce viral infections is to have good gardening practices and observe your plants on a regular basis. However, be aware that some nutrient deficiencies cause streaking in leaves and flowers. The continuing hybridization of viral resistant cultivars will also greatly help to control future problems for home gardeners and commercial growers.

Viral diseases affect all parts of the plant, the bulbs, roots, stem, leaves and flowers. So any bulblets or stem bulbils from an infected plant will also be infected. Only the seeds are not affected, so if you have a clump of infected lilies by pollinating the same plants with themselves a virus free stock can be achieved.

Diseases

Fungi are organism that live in or on plant tissues. They are related to mushrooms but do not necessarily grow the fruiting bodies that people know as mushrooms. The fungi take their nutrients from the plant and in turn destroy the host material. The two fungal diseases to cause the most problem are Basal Rot and Botrytis blight. Basal Rot is more destructive to the entire plant as it attacks the bulb, while Botrytis attacks the leaves stem and flowers. It weakens the plant over time and can eventually kill the lily. A few milder and less common fungal diseases are Black Scale Disease, blue mold, Cercosporella blight, root rot, rust, stump rot and Sclerotium. If you have a fungal infection in your plants it is best to destroy the plants than try to nurse them back to health as success is rarely achieved.

Basal Rot

Basal rot is caused by two different fungi, Fusarium oxysporum var lilii and Cylindrocarpon. Fusarium has a tendency to attack Asiatics while Cylindrocarpon attacks Orientals. Fusarium is the more serious of the two and can exist in the soil for three years without a host. Basal Rot can be recognized by a dark brown rot that extends into the scales from the basal plate. The scales may may fall apart as the scales become detached. The pathogen enters the plant from the roots, moving into the basal plate and then into the scales. The fungi reproduce by spores which can be carried in the soil or on the surface of garden or agricultural tools, packing material and bulbs.

Basal Rot usually shows symptoms of premature yellowing of foliage, stunting and premature senescence (drying) of the stalk. You may find that many scale bulblets form from the scales that have come off the basal plate, these however are usually infected. The outcome of Basal Rot is that the main bulb is usually destroyed, however the plant in efforts to continue may grow large numbers of stem bulblets which are not normally infected. So these can be dug up and planted in a different place to continue the variety. Do not replant anything if at all possible in the same spot as the infected lily for 4 or more years until the spores have died. Clean your garden tools before you plant the new bulblets.

Fusarium is most active in places where the soil temperature and moisture levels are high. In the north where soil temperatures are lower it is not as much of a problem.

It is the job of both home gardeners and commercial growers to try and use good cultural practices to control Basal Rot. The home gardener should only plant healthy clean bulbs and get rid of any bulbs that show signs of infection. One thing we home gardeners have a tendency to do is try to rescue or nurse plants back to health, this should be avoided with suspected Basal Rot. All scales and scale bulblets should be destroyed. Stem bulblets however can be saved. In heavily infected areas the soil should be completely replaces to a depth of 18 inches. I will not recommend or mention any fungicides to help control the problem, that is something you should research with local garden centers or Department of Agriculture.

The saying goes that the best defense is a good offence, will this is true in controlling infections from occurring. The best offence is prevention. Avoid fertilizers high in nitrogen, these cause rapid soft growth of the bulb. This makes then vulnerable to the fungi. Organic fertilizers such as manures and composts should be well aged and laid only as a top dressing or scratched into the first inch of soil. Well rotted manures and compost used as a top dressing or as a mulch will keep the soil cool. This will discourage Fusarium, which needs warm moist soils to thrive.

Another thing to do is control soil moisture. Plant lilies in well drained places. Avoid over watering in the hot summer months. Light well drained soils are better than heavy wet soils to reduce the moisture Fusarium needs to survive. This will not affect the bulbs much as they store moisture in the scales.

Fusarium  seems to like acidic soils, so adding some lime to increase the pH to a more neutral or alkaline level might be advisable in some soils.

Avoid damaging the bulbs during planting, transplanting or weeding. Any lesions could be and entrance for the fungi.

Commercial and amateur hybridizers need to concentrate of producing resistant varieties of new lilies. This is very important as chemical controls are becoming unavailable for various reasons, either the fungi is resisting the chemicals or they are removed from shelves by government regulations.

Botrytis

Botrytis is caused by two related species of fungi, B.elliptica and B.cinerea, both attack the above ground plant parts. B.elliptica is the more destructive of the two and both species can be found on the same plant at the same time. B.cinerea attacks the leaves, open flowers and seed pods in cool summer weather and late fall. The warm, moist coastal regions of the Pacific Northwest or western coast of Britain are often called Botrytis climates. Dry, cooler climates are rarely affected.

The fungus will over winter as small black sclerotia ( you will have to Google that one, its to long to explain) on the leaves of the previous year. These produce spores the in the summer that are wind blown or spread by splashing water. White spots on the leaves can be a first sign of Botrytis. These then become teardrop shaped on the upper surface, they are lighter in the margins and darker in the centers. As the attack continues the whole leaf collapses and decays.

Botrytis only attacks the surface of the plant and can spread to other surfaces or neighbouring plants as spores develop. Moisture is essential to the spread of the fungi so careful watering practices should be observed. It is better to use soaker hoses is better than a top sprinkler type watering system. Good air circulation is also important to dry the leaves quickly after watering or rain. Try not to get the leaves or the flowers wet. Keep an eye open for infected leaves and remove them in the morning when they are still wet. This will stop or in the least inhibit the spread of the fungi. Warm dry sunny weather helps to control the fungi.

Root Rot

Root Rot are usually in association with poor drainage, lack of soil aeration and planting in thick heavy finely textured soils like clay. The best way to control root rot is to have light airy soils that drain away water quickly so the roots do not sit in water for long periods of time. Roots need oxygen and being in water deprives them of this.