DISEASES

DISEASES

A) FUNGAL DISEASES

4. SCLEROTINIA CROWN ROT
Symptoms:
  • It is a frequent disease in bedding plants causing damping-off of seedlings or crown rot of older plants.
  • Under moderately warm and humid weather the rate of spread is more rapid and is easily identified as cottony growth of the fungus that develops in the lower parts of plants.
  • Later black sclerotia, upto 1/8 inch in diameter appear on leaves and stem.
Pathogen:
  • Sclerotinia sclerotiorum (Lib.) de Bary.
  • Tuberoid sclerotia are formed which are usually 5-15 x 3-5 mm in size.
  • Ascometa discs are 1-10 mm in dia. and ascospores are 9-13 x 4-6.5 µm in size.
Disease cycle and epidemiology
  • The fungus survives as sclerotia in the soil.
  • Cool and humid weather favours the disease development.
Management:
  • Use of certified disease-free seeds in sterile media and sterile containers, strict sanitation with regards to benches, pots, debris, discarding of the affected plants, and increased spacing of the remaining plants and careful watering is recommended.

6. ALTERNARIA LEAF SPOT

Symptoms:
  • The spots are more conspicuous on the undersides of the lower leaves on the plants, but the upper young leaves may also be infected.
  • Initally the infection appears as water- soaked area, less than 1mm in diameter on the lower surface of the leaf which may enlarge upto 6-10mm in diameter and become irregular and the necrotic spots usually limited by the large vein of the leaf.
  • Some of the spots have several concentric rings of darker brown tissues surrounding a dark center.
  • Severe infection may kill the leaf or cause considerable leaf fall.
  • Dead leaves shrivel and blacken and fungus sporulates profusely.
Pathogen: Three pathogens Alternaria tenuis Neergaard, A. alternata (Fr.) Keissler and Macrosporium pelargonii Ell. & Ev. are associated with the disease.
Alternaria tenuis:
  • Mycelium is septate and pale to olivaceous brown in colour.
  • Conidiophores arise singly or in small fascicles through stomata from the mycelium present in the dead centres of spots.
  • Conidia are olivaceous brown, smooth having transverse and few longitudinal septa.
Disease cycle and epidemiology
  • The pathogen(s) overwinter in infected plant debris.
  • Warm and humid climatic conditions favour the development of the disease.
Management:
  • Collect and destroy the infected plant debris.
  • Apply mancozeb (0.25%) or captan (0.2%) or chlorothalonil (0.2%) as foliar sprays as the disease starts appearing.
Other fungal diseases of importance are
i. Cercospora leaf spot : Cercospora brunkii(Ell.& Gall)
ii. Rhizoctonia root and crown rot: Rhizoctonia solani Kuhn
iii. Fusarium rot : Fusarium oxysporum Schlecht., F. solani (Mart.) Appel et Wollenw, F. nivale (Fr.) Ces and F. roseumLk.
iv. Damping-off : Pythium and Rhizoctonia spp.


B) BACTERIAL DISEASES

7. BACTERIAL BLIGHT
Symptoms:
  • The infected leaves either produce round leaf spots or large, angular dead area depending upon the variety.
  • Leaves, stem and cuttings are susceptible where the spots appear as small water soaked on undersides of the leaf having well defined margins, slightly sunken with 2-3mm in dia. and rarely coalesce together.
  • Later it is followed by necrosis and wilting of the leaves, which abscises or droops and hangs on the plant for a week or two.
  • The original spots turn brown to black and become hard and dry.
  • Stem rot symptoms has given the term black rot to the disease.
  • Vascular bundles in stems and branches of plants infected with the bacteria become brown to black within 2-4 weeks of infection.
  • At this stage one or more leaves on a branch will usually wilt and exhibit the angular necrotic symptoms.
  • Thereafter, pith and cortex turn black and the exterior portions of the stem become grey and dull.
  • The stem rapidly blackens and shrivels into a dry rot, leaving the fibers and epidermis intact and the rest as a disorganized non-viscous mass.
  • The roots become are blackened but not decayed and the plants are often difficult to uproot.
  • Symptoms if present on cuttings fail to root and slowly rot from the base upward.
  • The leaves wilt and often show the typical angular necrosis and wilt.
  • Stem become dull black-brown just like the stem rot phase.
Pathogen:
  • The bacterium Xanthomonas campestris pv. pelargonii (Brown) Dye, X. campestris (Pammel) Dowson causes the disease.
  • Bacterium is an aerobic, gram-negative, non-spore forming round rods, 1-1.5 x 0.5-0.7 µm in size, single or in pairs or sometimes in short chains, motile with polar flagellum.
Disease cycle and epidemiology
  • Bacterium can grow best at 25-27oC. The symptoms are more evident in cool conditions on plants and if exposed to warm conditions the disease resulted in sudden wilting and death of the plants.
  • The dissemination of the pathogen is carried out by splashing water either during propagation in the misted area or by routine overhead watering procedures.
  • Seed transmission is also an important mode of spread of the pathogen.
Management:
  • A thorough greenhouse clean-up is essential such as discarding all infected stock plants and debris, scrubbing down benches, clean up clay and plastic pots by steaming or sodium hypochlorite solution or formaldehyde soak for 10 minutes is essential.
  • Provide good ventilation in greenhouse with good cultural practices.
  • Prevent introduction of bacterium via cutting into a clean greenhouse by using culture-indexed cuttings and avoid excess crowding.
  • Dip the cutting knives or other instruments by dipping in 70% alcohol and flaming between each stock plant.

Other bacterial diseases reported on crop plant are:
i. Bacterial Leaf spot: Pseudomonas cichorii (Swingle) Stapp.
ii. Southern Bacterial wilt : Pseudomonas solanacearum E.F.Sm
iii. Bacterial fasciation : Clavibacter spp
iv. Crown gall: Agrobacterium tumefaciens (E.F.Sm. & Twn.) Conn.)


C) VIRAL DISEASES

8. GERANIUM LEAF CURL
Symptoms:
  • Irregular to circular, chlorotic areas 0.5-5mm in diameter appear on leaves which often cause ruffling, crinkling, malformation and dwarfing of the foliage.
  • The centers of the chlorotic spots may become brown with a chlorotic border and severely affected leaves become yellow and droop down.
  • Brown elongated corky raised necrotic area usually form on the petioles and stems and in extreme cases, the entire top portion of the plant may turn brown and die.
  • Symptoms are more prominent in spring months leading to disfiguring and dwarfing of the plants.
  • The young leaves of affected plants are wrinkled and deformed with slightly thickened hyaline spots which later become yellow and form small necrotic spots.
  • The virus is transmitted by white fly vector, plant parts, grafting and sap.
Last modified: Monday, 12 March 2012, 6:22 AM