1. PYTHIUM BLACK LEG
The disease primarily occurs on cuttings under propagation or may develop in plants at any age. The losses vary from traces to 100%.
Symptoms:
- The foremost symptoms of the disease are appearance of brown water-soaked areas at the base of cuttings or where stems have been otherwise wounded.
- The disease progresses rapidly several inches upto the stem, with affected areas becoming coal black.
- Leaves wilt as the rot advances upto the stem and eventually the plant dies.
- The coal rot symptoms distinguish the disease from the cutting rot phase develop in case of Botrytis blight, which is dull brown to dark brown in appearance.
Pathogen:
- Pythium spp.viz; P. irregulare Buisman, P. debaryanum Hesse, P. myriotylum Drechsler, P. splendens Braun.
- The pathogen forms filamentous inflated, spherical, ovoid to pyriform sporangia and the differentiation of zoospores takes place in vesicle.
- Oospores are plerotic or aplerotic.
Disease cycle and epidemiology:
- The spread occurs primarily due to the use of infested sand in the cutting benches, infected potted soil and infected plant cuttings.
Management:
- The incidence of the disease in commercial greenhouses can be reduced by the use of sterile propagating media, disease-free stock plants and strict sanitation practices.
- Avoid overhead watering to the plants and use of bottom heat to speed up the rooting process during propagation.
- Fungicides drench with etridiazole, fenaminosulf and metalaxyl (0.25%) are effective.
- Amendment with Trichoderma viride and Streptomyces sp. to P. splendens infested soil–less media delayed the expression of symptoms by 22-28 days.
2. BOTRYTIS BLIGHT
Symptoms:
- The fungus grows and sporulates rapidly on old and dying leaves, stems and dying floral parts.
- Infected petals wilt and drop on the foliage where they serve as a source of infection to leaves.
- Definite leaf spots occur under humid conditions in green houses, frequently the entire leaf and petiole get involved.
- The early fading or drying up of blossom is a common feature.
Pathogen:
- Botrytis cinerea Pers. ex. Fr. (perfect stage: Botryotinia pelargonii Roed).
- Conidia are aseptate, hyaline mostly 8-14 x 6-9 µm in size.
- The fungus also forms sclerotia just beneath the cuticle or epidermis.
Disease cycle and epidemiology
- The fungus survives either through the mycelium or sclerotia.
- The disease is more apparent during propagation and flowering.
- High moisture level in greenhouse predisposes the plants to infection.
- Wounds of any kind predispose tissues to infection and colonization by the pathogen.
- Pathogen may also become saprophytic but can seriously damage the healthy geraniums, particularly those tissues that are soft and succulent.
- The sporulation of the pathogen is maximum at 20-25oC but decreases with high temperature 30oC.
Management:
- Regulate the relative humidity levels in greenhouse that can be accomplished by proper spacing and watering and by ventilation and heating.
- It is also essential to evacuate the warm, moist air in the evening, replacing it with cool, dry air that is then heated to a desirable temperature.
- Proper sanitation, debris removal from the growing area reduces the level of disease spread.
- Grow tolerant disease varieties of geranium like Hawkeye, Waltztime, Hazel, Marian and Pearl Bailey.
- Fungicides such as Benomyl (0.05%), chlorothalonil (0.2%) or their combination can be used as protectants against infection.
- Captan (0.25%) with certain copper and sulphur mixtures (0.2%) and Vinclozolin (0.2%) alone is efficacious in protecting the plants.
3. RUST
Symptoms:
- The typical symptoms of the disease are white flecks, pinhead size usually on the upper surface of leaves while directly beneath these white flecks on the lower surface of the leaves are found small, white or chlorotic blister-like pustules.
- As the chlorotic spots enlarge, the pustules in the centre enlarge, erupt and become reddish brown with spores.
- Uredia are then observed in a more or less circular fashion around the original sorus.
- Most rust spots are formed on the lower surface of the leaves although they occasionally form on the upper surface.
- Heavily infected leaves turn chlorotic and defoliate.
- The spots are more apparent on leaves but also may develop on petioles, stipules and stems.
Pathology:
- Puccinia pelargonii-zonalis Doidge.
- The fungus produces subglobose to ovate, light brown uredospores measuring 24.5x22.0 µm, thin echinulated with two equatorial germ pores.
- Telial stage is comparatively very rare.
- The fungus is microcyclic; teliospores are 36-50 x 16-24 µm in size.
Disease cycle and epidemiology
- Heavy spore production and pustule formation occurred within 14 days of infection at 21oC.
- The germination of these spores are accelerated in water at an optimum temperature of 16oC although the infection is more at 21oC than the higher temperature of 27 oC.
- The fungus forms appressoria and penetrated through the stomata by intercellular hyphae and intracellular haustoria.
- The fungus overwinters in diseased leaves in form of uredospores.
Management:
- Use resistant cultivars for planting such as Crimson Fire, Yours Truly, Sincerity and Springtime Irene.
- Follow thermotherapy with a 24 hr water saturated air at 38oC or a 90 sec dip in water at 50oC, of course there is little injury to small expanding leaves with the hot water treatment but no effect on plant growth retardation is observed.
- Application of mancozeb (0.25%), tridimefon, oxycarboxin and triforine (0.05% each) fungicides at two weeks intervals give preventive or curative effect.
- Bitertanol (0.05%) can be applied as spray at 30 days intervals.
4. VERTICILLIUM WILT
Symptoms:
- The most common symptoms are bright yellow spottings on leaves, severe necrosis of shoot terminals or else extreme dwarfing of plants accomplished by foliar discoloration or wilting.
- The collapse of few leaves in the middle or upper portion of the main stem or side branches of the plant is noticed in some cultivars.
- At first, the petiole remain normal but after few days, they turn yellow and wilt followed by a gradual drying of the infected leaves and petioles resulting in leaf drop and defoliation of the affected area of the plant.
- In final stages the affected plants may show the dieback symptoms or drying tips of branches and brown to black discolouration of the main stem and branches.
- The browning is limited to the vascular tissues.
Pathogen:
- Verticillium albo-atrum reinke & Berthold and V. dahliae Kleb .
- Conidiophores, branches and phialides are commonly in verticals.
- Conidia are aseptate, hyaline, in slimy masses.
- V. albo–atrum does not form microsclerotia while V. dahliae forms true microsclerotia.
Disease cycle and epidemiology
- The pathogen(s) overwinter in the soil. Summer months are more conducive for the spread of the disease.
Management:
- Use culture-indexed plants as stock material or certified disease free seed, sterile soil or planting media and strict sanitation measures.
- Suppress the symptoms by drenching with Benomyl at 1.5 g a.i/4.5l at the rate of 200ml/plant.
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