PATHOLOGY

Diseases


Diseases of Greenhouse Ornamental Crops

Aerial Blight(Phytophthora parasitica)
Most susceptible plants: vinca.

This fungal pathogen causes a major disease in vinca in the landscape but it can also be a serious disease of greenhouse-grown vinca.

Symptoms. Initial symptoms of infection occur on leaves. Leaf infection is characterized by a rapid collapse of the leaf. Infection progresses to the leaf petiole and to the location of the attachment of the petiole to the plant stem. A brown, sunken stem lesion develops at the point where the petiole attaches to the stem. This brown stem lesion develops on the stem causing stem collapse. If wet conditions persist following plant infection, the fungus will grow to the base of the plant resulting in plant death. A unique feature of Phytophthora aerial blight is its decided aerial nature; this fungus rarely causes a primary root rot but causes massive damage to the aerial portion of the plant.

Control. Control of this disease has been very difficult and has proven impossible in many situations. Most effective control in the greenhouse will require that the plants are grown on benches, as far above soil as feasible. Remove symptomatic plants as soon as they become obvious on the greenhouse bench. Although foliar-fungicides have not been totally satisfactory, and have failed totally in the landscape for control of this disease, application may be useful in greenhouse situations if applied at the first sign of disease.

Bacterial Blight (Xanthomonas campestris)
Most susceptible plants: begonia, geranium, zinnia.

This disease can be devastating, causing loss of an entire crop. The pathogen is systemic so watch for symptoms and take immediate action upon detection.

Symptoms. Leaf spots begin as water-soaked blotches on leaves, frequently with large areas of chlorosis and browning at the leaf margins. An inverted "V" pattern commonly develops soon after initial symptoms occur; the "V"-shaped lesion usually turns a copper color as the infection proceeds. Leaf collapse is common as is the development of a soft, watery deterioration of the plant stem. Plant wilt and decline follows.

Control. The most important management strategy for this disease is to avoid introducing the pathogen into the greenhouse. Use disease-free, culture-indexed cuttings. Know the typical symptom to look for. There is no effective chemical control. Aggressive roguing of infected or symptomatic plants is important. Keep foliage dry; avoid overhead wetting/irrigation.

Bacterial Stem Rot(Erwinia carotovora pv carotovora)
Most susceptible plants: kalanchoe.

Symptoms. Infected plants develop a black, soil-line lesion that usually results in stem weakening, lower stem breakage, plant stunting, plant wilt and plant death. The soil-line lesion is usually very soft and mushy in texture.

Control. Take care to avoid wounding the plant stem during transplanting or when spacing/moving pots. Chemical controls have not proven effective.

Black Root Rot(Thielaviopsis basicola)
Most susceptible plants: pansy, vinca.

Black root rot can cause significant production losses in greenhouse crops. Although this fungal pathogen also has a very wide host range, the most serious problems occurr on pansy and vinca. Pansy and vinca plug infection has resulted in significant plant losses.

Symptoms. The black root rot fungus damages the root of the plant, effectively interfering with the root's ability to absorb nutrients. As a result of root injury, plants usually develop symptoms indicative of nutritional stress. Yellowing of the younger growth is a common symptom. Root examination of infected plants usually reveals a lack of healthy, white roots; infected roots are usually off-white, gray or black, depending on the stage and severity of infection.

Control. Control of black root rot can be difficult if the pathogen becomes established within the growing area. Pay strict attention to sanitation. Do not reuse plug trays or plastic pots. Store media in a location that is protected from contamination. Spot-check all plugs introduced into the growing area by carefully examining roots for healthy, white color. Stress has been shown to greatly enhance black root rot. Adverse temperatures, excessive moisture in the root zone, excessive levels of soluble salts and excessive use of fungicides or other plant production chemicals have all been implicated in intensification of black root rot. Because plugs are vulnerable to a number of stresses, all plugs should be planted as soon as possible after arrival.

Several fungicides have proven effective. Use preventatively or at the first sign of infection for effective control. An acidic pH helps to manage black root rot; a pH range of 5.5-5.8 can reduce black root rot development.

Botrytis Blight (Botrytis cinerea)
Most susceptible plants: exacum, geranium, impatiens.

Probably the most common and troubling greenhouse pathogen is the gray mold fungus, Botrytis cinerea. Botrytis can infect any above-ground portion of the plant. Wounded or stressed tissues are much more susceptible to infection. Botrytis can cause serious problems in geranium both as a flower blight as well as a stem/cutting rot.

Symptoms. Diagnosis is usually straightforward and is characterized by the obvious gray-to-brown fuzzy growth on infected plant tissue. This disease is much more common when relative humidity is high, when air stagnation is present, when temperatures are cool and where free moisture occurs on plant parts.

Control:

  • Avoid injury or stress to plants.
  • Encourage good air circulation and air movement within the greenhouse.
  • Space plants so as to encourage air movement around individual plants.
  • Use night heat plus ventilation at night to lower relative humidity within the greenhouse.
  • Keep temperatures above 60ÉC if possible.
  • Thorough greenhouse sanitation is MANDATORY! Remove infected plant material and plant debris.
  • Fungicide application may be effective if started early and if fungicides are rotated.
Crown and Root Rot (Phytophthora spp.)
Most susceptible plants: gerbera daisy, gloxinia, pansy

This disease is second only to Impatiens Necrotic Spot virus as a serious pathogen in gloxinia. Root and crown rot can also cause serious losses in gerbera daisy production. Infection can occur at any stage of gerbera production but seems to be more common after flowering begins.

Symptoms. Plants fail to grow adequately and usually remain noticeably stunted. The foliage develops an obvious off-color. Petioles may become infected where they attach in the crown area; as the petiole collapses, the attached leaf dies also. As the disease progresses, the entire plant wilts and dies. Phytophthora root and crown rot more commonly results in a rapid plant wilt, in which the plants appear normal and healthy one day but develop symptoms of rapid wilt and decline. The pattern in the flat or on the greenhouse bench is usually random but can be extensive, involving a large number of plants, especially if plants are not grown on benches. Overhead watering readily splashes the pathogen from plant-to-plant. Examination of the symptomatic plant will usually reveal a very wet root ball with significant root and crown discoloration and deterioration.

Control. Avoid growing plants on the ground or on mesh on the ground. Make sure that transplants are not planted too deeply: at the approximate same depth as the original plug not below the crown area. Avoid compaction of the rooting medium. Irrigate carefully; avoid overwatering. Drench-applied fungicides can be effective in managing this disease but should not be relied on to overcome poor cultural conditions.

Fusarium Stem and Root Rot (Fusarium solani/Nectria spp.)
Most susceptible plants: exacum

Infection by this fungal pathogen can cause rapid wilt and plant decline. This disease can easily be confused with Impatiens Necrotic Spot Virus and Botrytis stem infection.

Symptoms. Typical symptoms develop as off-color foliage initially, with a progressive desiccation and wilt of branch sections or commonly, one side of the plant. Eventually, the entire plant may yellow, wilt and die. Examination of the lower stem commonly shows a whitish fungal growth in association with an obvious lower stem rot. Small, reddish round balls, which represent the Nectria stage (the sexual stage) of the fungus are commonly observed.

Control. Remove infected plants as soon as symptoms are observed. Preventative drench-applied fungicides have shown good control of this disease.

Fusarium Wilt (Fusarium oxysporum f.sp. cyclaminis)
Most susceptible plants: cyclamen

Infection can occur at the seedling stage, with symptoms becoming obvious in older plants following an environmental trigger such as hot greenhouse temperatures.

Symptoms. Infection is usually characterized by chlorosis of the leaf tissue, collapse of the leaf petiole, plant wilt and plant death. Lower leaves usually wilt and yellow first, followed by the rest of the foliage. The corm usually remains firm. The most distinguishing diagnostic feature of infection is the obvious brown-black discoloration of the internal tissue of the corm that can be seen if an infected corm is split lengthwise.

Control. This disease is best controlled by strict sanitation. Remove infected plants as soon as symptoms develop. Fungicides have not proven effective.

Impatiens Necrotic Spot Virus (INSV)
Most susceptible plants: begonia, chrysanthemum, exacum, gloxinia, impatiens, vinca

This is one of the most serious diseases of greenhouse crops. INSV has a very large host range, numbering more than 648 different plant species. It is the number one disease of gloxinia and impatiens. INSV is transmitted by the thrips insect; it is not known to be routinely transmitted by any other means. One of the most frustrating features of INSV is that infection can result in a number of different symptoms.

Symptoms. Symptoms of INSV infection may include black ring spots (impatiens), black foliar lesions (impatiens, vinca, cineraria), chlorotic ringspots (exacum, gloxinia, cyclamen), veinal necrosis (gloxinia, Aphelandra, impatiens), stem lesions (chrysanthemum, exacum), distortion of young growth, stunting and plant wilt. These symptoms can vary with the stage of growth of the host and with a variety of cultural conditions.

Plants infected with INSV are systemically infected for the life of the plant. Although the plants are systemically infected, INSV can "compartmentalize" within its host, causing symptoms to occur only in a portion of the plant.

Control. INSV plants must be rapidly and thoroughly rogued from the production area to reduce infection to other susceptible plants. Monitor thrips activity routinely throughout the production area by the use of yellow or blue "sticky cards." Destroy all weeds, both inside and outside the greenhouse, as weeds can serve as both a reservoir of the INSV virus as well as a habitat for the thrips vector. Control thrips activity by appropriate management strategies, including insecticides when needed.

Powdery Mildew (Erysiphe cichoraceaum)
Most susceptible plants: begonia, chrysanthemum, gerbera daisy, kalanchoe, zinnia

Although most greenhouse crops can be infected by powdery mildew pathogens, each powdery mildew pathogen is specific to its host. The powdery mildew that infects gerbera daisy will not infect zinnia, etc. Powdery mildew is the most common disease of gerbera. Infection may start on the lower foliage and escape early detection if plants are not periodically monitored for this disease. Powdery mildew pathogens are readily disseminated in the air by air currents. Conditions that favor the development of powdery mildew diseases are moderate temperatures with high humidity. Some powdery mildew pathogens are enhanced by fluctuations between warm and cool temperatures but a relative humidity of 85% is generally needed for disease to develop.

Symptoms. Powdery mildew diseases are very common to a number of greenhouse crops and are easy to diagnose by the development of an obvious white powder on the plant surface. An atypical brownish scab-like symptom also may occur.

Control. Fungicides can be effective in managing powdery mildew diseases but take care to initiate applications at the first sign of infection and to rotate among effective fungicides. Include a systemic fungicide in the spray rotation.

Pythium Black Leg (Pythium spp.)
Most susceptible plants: geranium

Symptoms. Pythium black leg develops as a distinctive blackened deterioration of the lower stem, starting at the soil line. Infected stem tissue softens and deteriorates, damaging the plant's vascular tissue and interfering with movement of moisture to the leaves and other above-ground tissue. Plant wilt, stem collapse and plant death commonly result.

Control. Take special care during transplanting to avoid excessive planting depth and wounding of stem and root tissue. Avoid excessive soil compaction during the transplant operation. Water carefully; avoid overwatering. Drench-applied fungicides can be effective in controlling this disease problem.

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