Common diseases and disorders of Gardenias

A.R. Chase is a Plant Pathologist and President of Chase Research Gardens, Inc.
C.R.G. is a private research and consulting corporation specializing in ornamental plants.
Both hourly and contract services are available.
You can reach C.R.G. at P.O. Box 168, Mt. Aukum, CA 95656 Phone and Fax: 916-620-1624 - E-mail: MTAUKUM@AOL.COM


Bud drop
Pathogen - High heat and/or light and lack of water
Symptoms - The bases of the flower buds appear dry and turn tan to dark brown. Buds rapidly drop from plants without signs of fungal infection.
Control - Maintain adequate water especially when temperatures rise dramatically or when light level suddenly increases. Plants may be especially sensitive to dessication when conditions change from cool and dark to hot and bright as can occur during the spring months.

Iron deficiency
Pathogen - High medium pH and/or low levels of iron available
Symptoms - High medium pH can induce deficiencies of manganese, magnesium and most commonly, iron. This can result in interveinal chlorosis of young foliage. As deficiencies become more severe, necrotic spots will appear within the chlorotic tissue.
Control - Maintaining soil pH below 7 will improve the availability of iron and manganese and slow the leaching of magnesium. Application of the appropriate nutrient to the soil or potting medium will correct this problem, but plant response tends to be slow. Foliar sprays will hasten color improvement compared to soil application, but use low rates of application rates to avoid phytotoxicity from the sprayed materials.

Myrothecium leaf spot and petiole rot
Pathogen - Myrothecium roridum
Symptoms - Myrothecium leaf spot most frequently appears on wounded areas of leaves such as tips and breaks in the main vein which occur during handling. The leaf spots are watery and nearly always contain the black and white fungal fruiting bodies in concentric rings near the outer edge of the spot. They are seen on the leaf undersides. The presence of these bodies is good evidence that the cause is Myrothecium. Newly planted cuttings are especially susceptible to this disease since they are frequently rooted under mist conditions.
Control - Iprodione (Chipco 26019 50WP) provides some disease control. Preventive treatments to newly rooted cuttings may be required. In addition, mancozeb (Dithane M-45) provides good control of the leaf spot on some plants. Avoid wounding leaves and keep the foliage as dry as possible.

Phytophthora stem rot and leaf spot
Pathogen - Phytophthora parasitica
Symptoms - This disease occurs primarily on plants grown in or on the ground. Leaf spots are initially small and water-soaked, with irregular margins. They may become tan and papery if conditions are dry or their centers may fall out if conditions are wet. Stem rot usually begins at the soil line where the stem becomes soft and watery and lower leaves turn yellow. Eventually, the area becomes sunken and a cavity may form and result in lodging of the stem.
Control - The combination product of thiophanate methyl and etridiazole (Banrot), etridiazole (Terrazole and Truban), and metalaxyl (Subdue 2E) provide disease control. Growing plants on raised benches, away from the natural source of infection (the soil), is the best way to avoid this disease. Due to similarities between this and several other diseases, diagnosis must be confirmed by a diagnostic laboratory before optimum control strategies can be chosen.

Rhizoctonia aerial blight or leaf spot
Pathogen - Rhizoctonia solani
Symptoms - Rhizoctonia aerial blight occurs primarily during the summer months. Disease development can occur in less than a week, so plants should be checked carefully and frequently. Brown irregularly shaped spots form anywhere in the foliage, but most commonly within the crown of the plant which is often wet. Sometimes the first spots form near the top of plant confusing the source of the disease (the soil). The spots spread rapidly and cover the entire plant with the brown weblike mycelium of the pathogen.
Control - Since this pathogen inhabits the soil both the roots and the foliage of the plants must be treated to provide optimal disease control. A combination drench-spray will best accomplish this. Thiophanate metyl compounds (Cleary's 3336 or Topsin M) and chlorothalonil (Daconil 2787) are effective in controlling this disease.


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