|
By Marilyn H. S. Light Wed. Jan 14, 2004 |
|
Your orchid is failing to thrive, has unsightly blotches, tiny speckles, or deformed flowers. What is the problem? Read almost any book about orchid culture or search key phrases such as orchid+diseases and orchid+pests via the web and you will find as vast an array of diagnostics and solutions that you could imagine. Read a book or peruse the internet, and you will face another challenge. Is what you have in hand that which is pictured or described? For example, your precious Masdevallia has leaves that look as if every disease imaginable has attacked with vengeance. What is the problem? picture Just how potentially damaging is this 'problem' to the orchid collection? What measures should be employed to control it? Is the best approach, 'toss' and try again? Should you spray first and ask questions later or should you first try to outline why you think there is a problem in hope of developing a possible solution? The 'good' news! When 'symptoms' do not necessarily mean serious trouble. Some orchids have naturally spotted foliage while others have dark hairs or scales. For example, the leaves of Maxillaria reichenheimiana have white spots. The bracts and leaves of Rudolfiella picta are marked with mauve speckles that fade slowly with age. These can become confused with disease symptoms especially in older leaves where there is other unrelated blotching present. picture Every spot, blotch, eruption or discoloration is not necessarily a sign that an orchid has been stricken with a deadly disease. The awful appearance could be the result of a natural process or an environmental injury. This older leaf of Rudolfiella picta has many irregular black spots resulting from physical damage several years previous. picture Through close examination of the Rudolfiella leaf, we can see the reddish cell inclusions in the central part of the upper sample. These spots give the blotch its coloration when viewed from a distance. We can just make out the coloration when looking closely at the leaf from above. (This image may be a little dark) picture When viewed through a microscope, the inclusions are immediately apparent. picture Pseudobulbs of some orchids such as Cymbidiella pardolina may darken with age as part of a natural process (T Watkinson, personal communication). In this photo kindly provided by and Copyright Tony Watkinson, we see a flowering specimen of the Cymbidiella. picture When we look closely at the young and older pseudobulbs of the same plant photographed by and Copyright Tony Watkinson, we can clearly see the darkening process on pseudobulbs. Thank you Tony. picture Ed_in_Sat MarilyninOttawa There are instances where sudden darkening of plant parts, especially when the growing shoot can be easily pulled out or there is leaf drop, or a bad smell emanates from the discolored parts, is a sign that something is seriously wrong. Here we see a new shoot of Catasetum with a potentially deadly bacterial rot. picture How can you be certain that what you are observing is some dreaded ailment and not a natural or a benign process? Before consulting a fellow hobbyist or specialist, book, website or diagnostic service, assemble what you already know and have observed. STEP 1 - Answer the following questions about the symptoms. 1 - Is the affected area small or large? What does it look like? In this example, a leaf of Encyclia sp., we can describe the leaf spots as being dry, of various sizes, irregular, brown with a halo, and clustered in the upper part but not the tip. There are also some translucent areas. There is no smell although there are a few scale insects present. picture 2 - Is the area confined to the growing point/new shoot, pseudobulb, flowers, old leaves, new leaves or to a particular part of a leaf such as the tip? In this example, the second bud of our favorite Paph. bellatulum has withered and turned brown. picture 3 - Have the symptoms suddenly appeared on one or on several plants at the same time especially if they are all located in one part of the growing area? Many of the Masdevallias in this collection are showing the same leaf blotches. picture 4 - Is the affected area enlarging rapidly (doubling in size over a few days or less)? Crown rot of Phalaenopsis and shoot rot of orchids like Catasetum progresses very rapidly. The shoot rot of the Catasetum will progress to death of the growing point and the entire shoot in less than a week. picture 5 - Are leaves suddenly turning yellow and dropping? It is normal for the lowest or oldest leaves of many orchids to yellow and fall. This may happen when plants are brought indoors after a summer sojourn. It is likewise normal for some orchids such as Catasetum, Cypripedium, Lycaste, and Pleione to shed their leaves about once a year. It is not normal for a Phalaenopsis to lose most of its few leaves over a few months. 6 - Is the affected area soft or oozing fluid? Bacterial decay is moist since the bacteria are producing enzymes which digest cell walls permitting the infection to spread rapidly giving them access to new sources of food. The lesion may resemble a thinly covered blister but very fragile and liable to rupture, releasing millions of infective bacteria with every drop. picture 7 - Does the affected part smell bad or of rotting fruit? Bacterial ailments and some kinds of fungal disease such as Black Rot caused by Phytophthora sp. have an unmistakable odor that is somewhat sweet and fetid. Some other symptoms that may or may not be false alarms: Black spheres The tiny black spheres (peridioles) stuck to the surfaces of leaves and to the inside of light reflectors are the reproductive bodies produced by the Artillery Fungus, Sphaerobolus stellatus, that lives harmlessly in moist bark or coconut husk in orchid pots. The spheres may look ominous but are nothing to worry about at least not for the orchid grower. They seem to be more of a problem to homeowners whose white vinyl siding becomes peppered with the sticky spheres ejected by the parent fungus from up to 20 feet away where it is living happily in garden mulch. picture The spheres are quite soft and can be crushed gently to reveal their contents. picture Under high magnification, we can see the spores contained inside. picture Physical damage. The small, isolated spots that appear on older foliage could be part of the natural aging process or the result of physical impact or physiological damage such as sunburn, overheating or a chill. Brown leaf tips. Browning leaf tips are often suggestive of root/watering problems as with this Phragmipedium. picture The dying leaf tips may become colonized by opportunistic fungi which could eventually become a problem but since cultural technique is likely the cause, correcting that will usually stop the problem in its tracks. Here we see Anthracnose attacking leaf tissue which had first been compromised by inadequate watering and root loss. Before the fungus appeared, the tips first became light yellow, then papery and only after several months did the fungus become evident. picture Spotted flowers. Cool, humid conditions are a breeding ground for Botrytis, a fungus which attacks flowers, disfiguring beautiful flowers with scattered black dots. Improved ventilation and some nighttime heating will minimize or eliminate the problem. Black Sooty Mold. Black Sooty Mold growing on leaf surfaces is unsightly but harmless. The presence of Sooty Mold is symptomatic of scale insects or aphids feeding on the leaves above. The insects produce sticky secretions (honeydew) which drips to the leaves beneath and there supports the growth of the sooty mold. Control of the pest will ultimately eliminate the symptoms. Phalaenopsis Leaf Blotches. The variously shaped yellowish blotches that appear on the leaves of Phalaenopsis can be particularly worrying and sometimes rightly so. The problem is that there can be many causes including physiological damage, 'microfungus' and virus infection, all of which have to be investigated. The arrow points to a yellowish area of the leaf where the problem is starting to manifest. There are some other suspicious areas but many marks result from physical damage to this cared for specimen. picture More worrying still would be the appearance of blackened shoots and collapsing pseudobulbs or soft, smelly oozing leaf lesions that enlarge rapidly signalling that an aggressive pathogen, bacteria or fungus, is attacking the orchid and requires your immediate attention. Here, the Black Rot fungus, Phytophthora, has attacked a new shoot of a Cattleya. The infection can progress via the rhizome to attack other pseudobulbs, the rhizome and roots, thus ultimately killing the plant. picture Viruses TIP! Orchid plants infected with virus may or may not exhibit symptoms but all infected plants are potential reservoirs of infection. Since there is no sure way of visually determining whether or not a plant is infected, since there is no known cure, and because plants carrying virus but without symptoms remain a threat to the collection, always use sterile cutting tools when dividing or potting plants and when cutting flowers. We can see symptoms of virus infection when they are apparent in this excellent publication produced by the University of Illinois Extension Department. button Does everyone understand the concept of describing symptoms? Step 2 - Answer the following questions about culture. 1 - Have the affected plants been recently acquired? There is growing concern about the spread of plant diseases. Quarantine newly acquired stock. Also, newly potted plants may bloom but be unable to sustain a blooms or open all the buds as in this recently imported Paphiopedilum where the second bud has aborted development. picture 2 - Has there been a recent abrupt change in growing conditions? Have the plants been recently moved from one residence to another or to a different area within the greenhouse? 3 - Have the plants been overheated or chilled? Heat and cold exposure can sometimes cause an immediate or a delayed response. Cool-growing Masdevallias exposed to high temperatures can later suffer the dreaded leaf drop and an untimely passing and there is nothing the grower can do about that except avoid making the same mistake again. picture 4 - Have the plants been exposed to paint fumes, smoke or household cleaning products? Some orchids are particularly sensitive to organic solvent fumes or when sprayed with a pesticide containing a solvent. Within 24 to 48 hours of exposure, leaves of sensitive species such as Epidendrum pseudepidendrum, Psychopsis papilio and Dendrobium moschatum will yellow and the plants can die. 5 - Has it been overcast for several consecutive days or have the plants been continuously wet for two or more days? Poor growing conditions, too little light, inadequate air movement and high humidity are always a recipe for disaster. Weakened plants may not be able to resist the invasion of even the most common micro-organisms. 6 - Has the rainy season recently begun or has there been a wind storm/hurricane when the plants were buffeted by gusts? Plants bruised during transit or because they were tossed about during a storm will have open wounds which are an ideal environment for disease to take hold. Diseases, especially the really damaging ones, do not arise spontaneously. The causative bacteria, fungi and viruses are imported into a collection with new acquisitions, by insects, by wind and water from sources outside the growing area, by cross contamination with other plants in transit or at a show, or even by ourselves when we unwittingly handle diseased material and then handle our plants without first washing our hands. When plants appear as if they are afflicted with a terrible disease, a grower should think about what might have led to this situation before reaching for the sprayer. Are there any questions or comments regarding the 'culture' questions? art,simpsonville_sc N_Calif_Kathy MarilyninOttawa N_Calif_Kathy STEP 3 - Consult an expert Orchid clubs are often the first place a worried hobbyist goes to find out more about their problem. When planning to consult a club member, always seal the specimen from contact with other plants. Place the sample in a sealed container such as a plastic bag. It is usually easier to identify a problem if the plant is still alive. I wonder how many clubs have an 'orchid doctor'? N_Calif_Kathy MarilyninOttawa Understand that a fellow grower may only be able to suggest an answer to a problem and that more detailed investigation and confirmatory tests may be needed. State, county, provincial and national governments as well as certain universities and grower associations may offer a pest and disease diagnostic service to farmers, greenhouse producers of commercial crops and often also to the public, usually for a fee. John in Arcadia Ca MarilyninOttawa One way that the causative organism can be revealed is by leaving an actively infected plant piece in a sealed bag. Under the high humidity conditions in the bag, the fungus will often grow out as tufts of white growth (mycelium) and this when viewed under the microscope will reveal the diagnostic spores. Here we see what grew out of an infected pseudobulb . The fungal thread (hypha) is branched and producing asexual spores by budding. You can see an individual spore (S) and a potentially confusing object (R) which is a calcium oxalate crystal which can be found in orchid cells. picture Once the problem is identified, then control measures appropriate to the plant and place can be suggested. As always, there is never a substitute for good culture including greenhouse sanitation and an observant and caring owner. In the next part of this series on orchid pests and diseases, we will look at some specific orchid diseases, their diagnosis and control measures. Some diagnostic services around the world In the United Kingdom, a horticultural diagnostic service is provided by the Central Science Laboratory. button The Royal Horticultural Society Advisory Service has a members-only service. button GrowHelp Australia provides a problem-solving diagnostic service for horticulture businesses including cut flower producers. button In the USA, a wide range of resources is available including the Extension Departments of some universities. Here are a few possibilities. Cornell University (NY) button University of Florida (FL) button University of Georgia (GA) button In preparation for next month's session, you might wish to look at the following sites. Orchid Diseases in the Northern Territory (Australia). J. Duff and A. Daly, 2002. AgNote 568, No. 13. button Pests & Diseases Section of the Philippine Orchid Society button One thing we must be very careful of is exposure of our collections to new pests and diseases. We will discuss this in more detail next session but there is an alarming increase of bacterial disease of Dendrobium and of several less common fungi afflicting Phalaenopsis. Whenever we bring new plants home, quarantine is essential. Art was quite correct when he commented on the impact of growing conditions on orchid health. As long as the less than ideally raised plants are not too susceptible to whatever organisms they are exposed to, and there are no really virulent disease organisms present, then the plants will survive virtually unscathed. Kathy told us about some of her challenges. Are there any other specific problems that you all might have for discussion next session? N_Calif_Kathy John in Arcadia Ca MarilyninOttawa John in Arcadia Ca N_Calif_Kathy MarilyninOttawa Carole MarilyninOttawa Carole Gaile MarilyninOttawa Gaile MarilyninOttawa Gaile MarilyninOttawa Jade Carole MarilyninOttawa Jade MarilyninOttawa |
|
Prechat Handout Orchid Diseases: Part I - A reasoned approach Marilyn H. S. Light Your orchid is failing to thrive, has unsightly blotches,
tiny speckles, or deformed flowers. What The 'good' news! When 'symptoms' do not necessarily mean serious trouble Every spot, blotch, eruption or discoloration is not necessarily
a sign that an orchid has been
STEP 1 - Answer the following questions about the symptoms. 1 - Is the affected area small or large? What does it look
like? Tiny black spheres stuck to the surfaces of leaves are the
reproductive bodies produced by the TIP! Orchid plants infected with virus may or may not exhibit
symptoms but all infected plants
Step 2 - Answer the following questions about culture. 1 - Have the affected plants been recently acquired? Poor growing conditions, too little light, inadequate air
movement and high humidity are always Orchids are quite resilient but can react to environmental
insult such as a chill by dropping
STEP 3 - Consult an expert Orchid clubs are often the first place a worried hobbyist
goes to find out more about their
Interesting Reading Orchid Diseases in the Northern Territory (Australia). J.
Duff and A. Daly, 2002. AgNote 568, Pests & Diseases Section of the Philippine Orchid Society Artillery Fungus - Sphaerobolus sp. |
