FUNGAL DISEASES

FUNGAL DISEASES

1. POWDERY MILDEW:
This is a very serious disease of this crop both in open and protected structures and affects both quality and quantity of cut flowers. Severe mildew growth reduces leaf growth, the aesthetic value of plants, photosynthetic efficiency and there by plants growth and stability of cut flowers.

Symptoms:
  • Small, discrete lesions appear on the stems, leaves or sometimes on the flowers (Plate 1, 2).
  • Production of numerous conidia gives the affected plants a white powdery appearance.
  • Newly unfolded leaves are most susceptible, becoming increasingly resistant with age.
  • The young growing tips are completely covered by the mildew.
  • As the cells are killed, dark spots may be noticed on the infected leaves.
  • Infected flower buds do not open.
  • The petals may become discoloured, dwarfed and finally die.

25.1

Pathogen:
  • The disease is caused by Sphaerotheca pannosa (Wallr. Ex Fr.) var. roseae Wor.
  • The appendages on cleistothecia of S. pannosa are vestigial or lacking and its ascocarps are usually embedded in felt like mycelium.
  • Each cleistothecium contain single ascus which bear usually 8 hyaline and elliptical ascospores.
Disease cycle and epidemiology:
  • Infection of dormant buds is the primary mode of perennation.
  • Conidia are produced at the primary infection sites readily dispersed by air movement.
  • Warm and humid weather with cool nights favours the development of the disease.
  • A temperature of 21oC and relative humidity ranging between 97-99% is quite effective for the spread of the pathogen.
Add animation no of rose
Management:
  • Pruning of infected shoots at the end of the season and burning of these shoots will help to prevent overwintering of the fungus.
  • Lowering of the night humidity by fans and /or venting or heating is recommended for greenhouse roses.
  • Spraying the crop with fungicides like hexaconazole (0.05%), difenoconazole (0.03%), triadimefon (0.075%), fenarimol (0.04%) or carbendazim (0.1%) effectively check this disease.
  • Use resistant sources, if available.
  • Biological control of pathogenic fungus can be obtained by applying hyperparasites like Sporothrix floculosa, Penicillium sp., Cladosporium sp., Cephalosporium sp. and Tilletiopsis pallescens.

2. BLACK SPOT
It has also been called as leaf blotch, leaf spot, blotch, rose actinonema and rose leaf asteroma and star sooty mould. This disease is considered as most troublesome disease of garden roses worldwide including India.

Symptoms:
  • Brown to black, nearly circular spots (2-12 mm in dia.) often has fringed margins with a feathery hyphal growth which appear on both sides of leaves (Plate-3).
  • The affected leaves turn yellow and drop prematurely due to the formation of ethylene which is more in green leaves infected with this disease.
  • Other above ground parts like fruits, sepals, stipules, petioles and peduncles may also develop similar spots.
  • Infections on petioles and stipules are usually small and inconspicuous while petal infection usually causes formation of small, reddish spots accompanied by distortion of the surrounding tissues.
  • Close examination of the lesion reveal the presence of small black acervuli which may be distributed over lesion surface or found in concentric circles.

25.3

Pathogen:
  • Diplocarpon rosae Wolf (Ana: Marssonina rosae (Lib.) Lind.).
  • The apothecia are globose to disc shaped, subcuticular, radiate and are 100-300 µm in dia.
  • Asci are inoperculate, oblong, and cylindrical, short stalked, 70-80 x 12-18 µm and contain a pore.
  • Each ascus contains 8 ascospores which are oblong, elliptical, hyaline, and unequally two-celled, constricted at the septum and are 20-25 x 5-6 µm in size.
  • The paraphyses are numerous, filiform, septate and enlarged at the tips.
  • Conidia are 18-25 x 5-6 µm in size, hyaline, oval to elliptical with single septation and are formed in an acervulus.
  • Acervuli are formed on leaves and young shoots during the summer, while apothecia and spermogonia are borne on decaying overwintering leaves.
Disease cycle and epidemiology:
  • The fungus perennates on overwintering rose leaves and infected stem tissue.
  • Although the perfect stage has been recorded in certain geographic areas, but their role is not understood in the pathogen life cycle.
  • For rapid spread of the disease, temperature ranging between 0-33oC, with optimum temperature (24oC) and more than 85% relative humidity is favourable.
Management:
  • Removal of overwintering inoculum by eliminating infected leaves is helpful in limiting the inoculum spread.
  • Avoid overhead irrigation and dense planting to allow good air circulation through leaf canopy and do not allow leaves to remain wet or high humidity for more than 7-12 hours.
  • Sprays of Bordeaux mixture, wettable sulphur, carbamates, ergosterol-inhibiting fungicides like triforine, benomyl-zinc, chlorothalonil, mancozeb provide effective control of this disease.
  • Jawahar and Pusa Sonia varieties of rose are resistant to black spot disease

3. GREY MOULD
Symptoms:
  • On leaves, water soaked lesions are formed which spread very fast.
  • On flowers, infection developed in form of water soaked lesions causing premature fading, discolouration and dropping.
  • The infection is gradually seen in the inner whorl of the flowers.
  • This disease prevents the blooms to open.
  • Buds turn brown and decay.
  • Individual petal turns brown and shrivels.
  • Infected parts may be covered with grey to greyish brown mycelial growth which often sporulate profusely and spread fast coinciding with the period of senescence.
Pathogen:
  • The disease is caused by Botrytis cinerea Pers. ex Fr.
  • Mycelium is septate, produce branched conidiophores which are 2 mm or more in length and 16-30 µm thick on which conidia of 6-18 x 4 x 11 µm size are formed, are single celled, ellipsoidal, hyaline to pale brown.
  • Sclerotia of variable shapes and sizes are produced.
Disease cycle and epidemiology:
  • The pathogen perennates either in the form of mycelium or sclerotia in the infected plant debris.
  • The conidia are airborne and maximum disease occurs during June and July.
  • Prolonged periods of free moisture favour infection.
  • For conidial germination, 18 to 25oC temperature and relative humidity between 93 to 100 per cent is optimum.
Management:
  • All fallen leaves and other plant debris should be collected and destroyed to reduce the primary inoculum level.
  • In greenhouses, proper ventilation should be maintained to prevent formation of free water on the plants.
  • Spray the crop either with carbendazim (0.1%) or thiophanate methyl (0.1%) or benomyl (0.1%) or iprodione (0.1%) and repeat at weekly interval.
  • The postharvest development of this disease in cut flowers can be suppressed when flower buds are sprayed with 1 mM solution of giberellic acid.

4. RUST
The disease is of worldwide distribution. Heavy losses occur due to premature defoliation.

Symptoms:
  • On upper leaf surface, minute, orange pycnia appear.
  • Yellow to brown pustules impart rusty appearance to the shoots.
  • The fungus produces aecial stage in spring.
  • The aecia are prominent as yellowish lesions on the lower surface and the spores get released as the soral membrane fragments following the rupture of the epidermis (Plate-4).
  • They are erumpent, bright orange, usually surrounded by a distinct but narrow chlorotic area.
  • Aecia and pycnia also appear on petioles and young woody stems.
  • After aeciospores infect the leaves, the uredinial stage starts forming as a whitish fleck on the lower surface of the lesion.
  • Mature uredinia are reddish, orbicular-shaped and contain orange coloured urediniospores which are released about 14 days after the leaf become infected.
  • These spores continue to cause new infections throughout the summer.

25.4

Pathogen:
  • Several species of Phragmedium are responsible for this diseases but P. mucronatum (Pers. Ex Pers.), P.fusiformie J. Schrot, P.tuberculatum, P. rosae-pimpinelifoliae Dietel are important.
Disease cycle and epidemiology:
  • The rust fungus overwinters either in the form of teliospores or as mycelium in the infected stems.
  • The optimum temperature range for aeciospores and urediniospores germination lies in between 15 to 21o C while teliospores are produced at 18o C.Wind helps in release of rust spores and their short and long distance dispersal.
Management:
  • Remove infected leaves and stem infection near the crown and destroy them.
  • Perform severe pruning to destroy the stem infection.
  • Spray the plants with dithiocarbamates, sulphur fungicides, oxycarboxin or ergosterol biosynthesis inhibiting fungicides.
  • Hyperparasites such as Darluca fulvum, Tuberculina vinosa and Verticillium lecani can be exploited as biocontrol agents.

Other minor fungal diseases:
i. Die-back: Diplodia rosarum Fr., Botryodiplodia theobromae (Pat.), Colletotrichum gloesporiodes (Penz.) Penz. & Sacc.
ii. Anthracnose: Sphaceloma rosarum (Pass.) Jenkins (perfect stage-Elsinoe rosarum Jenkins and Bitanc)
iii. Downy mildew: Peronospora sparsa Berk.
iv. Verticillium wilt :Verticillium albo-atrum Reinke & Berthold and V.dahliae Kleb.
v. Black mould: Chalaropsis thielavioides Peyr.
vi. Crown canker: Cylindrocladium scoparium Morg. and C. floridanum Sob. & Sey.
vii. Brown canker: Cryptosporella umbrina Jenkins (Imperfect stage-Phomopsis spp.)
viii. Stem and graft canker : Coniothyrium fuckelii Sacc.; perfect stage- Leptosphaeria coniothyrium (Fckl.) Sacc.)
ix. Colletotrichum leaf spot: Colletotrichum capsici (Syd.) Butler
x. Cercospora leaf spot : Cercospora rasicola Pass.
xi. Phytophthora stem blight :Phytophthora megasperma Drechsler
Last modified: Thursday, 24 January 2013, 5:52 AM