Leaf rust

Leaf rust
    • Coffee leaf rust caused by Hemileia vastatrix Berk. & Br. is one of the most important diseases of tropical plants.
    • It has been considered as one of the classic diseases, since it ruined the economy of Srilanka, growers had to abandon and shift to the Tea cultivation and the social habit of the people also changed from coffee drinking to Tea.
    • Besides India, coffee rust has been reported from 50 other coffee growing countries.
    • Leaf rust disease was noticed in India during 1869 and in the early years of infection it caused severe damage to the flourishing plantations sending the coffee production industries staggering down for control measures were not available during that time.
    Causal organism
    • The causal organism of leaf rust Hemilia vastatrix has been classified under
      • Class- Basidiomycotina,
      • Sub- class-Teliomycetidia,
      • Order- Uredinals
      • Family- Pucciniaceae.
      • In nature, coffee leaf rust fungus produces only the uredinial, telial & basidial stages. But the perpetuation of the fungus in nature is only through anomorphic uredospores.
      • The teliospores are produced only during unfavourable conditions which germinate in situ under favourable conditions to produce basidiospores which are apparently functionless on coffee.
      • Alternate hosts have not been reported so far. Pycnial & aecial stages of the fungus have not been noticed both under natural or controlled conditions.
      • Uredospores act as secondary source of inoculum.
    Symptoms
    • Coffee rust pathogen attacks mainly the leaves & very rarely the young branches.
    • Initially pale yellow circular spots measuring 2-4mm in diameter appear on the lower surface of leaves which later turn orange yellow powdery mass of urediniospores.
    • These spots enlarge & become more irregular as they coalesce with the adjacent spots.
    • With the aging, the central portions of the spots become brownish and necrotic, whereas the middle zones of the lesion continue to sporulate.
    • Severe infection can cause heavy defoliation & dieback of branches.
    • Crop loss
    • Rust affects the berry yield in many ways, by reducing the photosynthetic area, by occupying leaf area, inducing defoliation and reducing vigour of the plants due to altered physiology.
    • Depending on the severity of disease not only fewer flowers are formed but the flowers and fruit fall prematurely and the remaining berries do not develop to their maximum size.
    • In severely infected cases, the pathogen may causes losses up to 50% foliage and 70% loss of coffee berries.
    Coffee leaf rust
    Favourable factors & disease progress/Epidemiology
    • Wet weather with wind during May to November,
    • Intermittent and sun shine, mist or rains during dry weather from Nov-Mar. A thin or no over head shade is also the favourable factor for disease
    • Under suitable conditions, disease make its appearance after blossom showers during Mar-Apr as the foliage present at that time are mostly of previous season and on which the fungus remains dormant.
    • Such leaves during the dry weather sporulate during the dry weather to form fresh urediniospores.
    • The disease reaches in peak form from Sep-Nov & results in severe defoliation.
    • Mode of spread
    • One lesion produces 1.5 lakhs uredospores which are spread through rain splash and wind.
    • Many animals can also carry spores over long distances.
    • Infection requires the presence of water for uredospore germination and it enters through stomata which are on the underside of the leaf.
    • Epidemics takes place during the rainy season because of the water requirement.
    Life cycle

    Management
    Cultural management
    • Various agronomic practices like spacing, shading, fertilizing & pruning operations reduce the rust infection level.
    • Spacing:-Increased plant spacing decreases relative humidity & thus the relative incidence of leaf rust. Therefore avoid dense planting to minimize the rust incidence.
    • Shade:- Leaf rust incidence is generally lesser under medium shade than in open or thin shade.
    • Fertilizer:- Maintain vigour of the bush during crop development stage by applying balanced fertilizers, as plant health is one of the major factors that influence the rust development.
    • Pruning:- Pruning of dry & unproductive branches is done to reduce the die-back of branches and facilitate formation of new branches. It is mostly recommended before the blossom showers to reduce the initial inoculums level.
    • Biological control
    • Verticillium hemileiae, the parasite of rust spores occur mostly during rainy season when there is high relative humidity near saturation.
    • Role of a biological control agent is very limited due to its failure to establish even for a short period under low humidity.
    • Resistant varieties
    • Twelve cultivars of Coffee Arabica (Sln. 1 to Sln.12) were evolved at CCRI..
    Chemical control
    • Copper based fungicides especially freshly prepared Bordeaux mixture (0.5%) has been found effective & economical for the control of leaf rust under field conditions.
    • Systematic fungicides like oxycarboxin, carboxin, pyracarbolid, triadimefon, propiconazole, hexaconozole & epoxyconazole have curative & eradicant effect on rust pathogen.
    Spray schedule for the control of leaf rust
    • Bordeaux mixture (0.5%) pre- blossom (Feb-Mar), Pre-monsoon (May-June), Post-monsoon (Sept-Oct).
    • Triadimefon (0.02%) Pre-monsoon (May-June), Post-monsoon (Sept-Oct).
    • Hexaconozole (0.01%) Pre-monsoon (May-June), Post-monsoon (Sept-Oct).
    • Propiconazole (0.02%) Pre-monsoon May-June), Post-monsoon (Sept-Oct).
    • Oxycarboxin (0.03%) Pre-monsoon, Mid-monsoon (July-Aug), Post-monsoon (Sept-Oct).
    • Epoxyconazole (0.0026%) Pre-monsoon (May-June), Post-monsoon (Sept-Oct).

Last modified: Friday, 22 June 2012, 11:22 AM