Modern concept of plant disease management is based on the ecological principles and involves the integration of different control methods in disease management. Integrated disease management (10M) in turn is a component of agro-ecosystem management technology for sustainable crop production. 10M has been defined as disease management system that in context of associated environment and population dynamics of the pathogen species utilizing all suitable techniques and methods in compatible manner as far as possible to maintain the pathogen popJllation below threshold level. The purpose of 10M is not only to achieve the immediate object of preventing losses caused by the pathogens but also with a broader perspective to minimize the impact of pesticides in the environment.
The concept of 10M implies that the agro-ecosystem should be so adjusted to reduce the pathogen population that contains the crop loss. Some control methods are based on natural phenomenon namely host resistance and biological suppression of pathogens, while use of pesticides and cultural methods are inducted into the natural systems. Though the package recommended in coffee is not designated as IDM, most of its components are ge.nerally adopted for effective management of leaf rust.
For the intelligent and successful tackling of any plant disease it is most essential to have some basic knowledge about the causal organism, its damage, favorable factors for disease development. Accordingly, some basic information about coffee leaf rust is briefed before describing the individual component of the 10M procedures to be adopted in rust management.
History and Importance of leaf rust
Leaf rust caused by Hemileia vastastrix Berk.&Br. on arabica coffee is one of the important and classical diseases, which not only ruined the economy of Sri Lanka but also changed the social habits of both coffee producing and consuming countries that changed drinking habits from coffee to tea. Coffee rust has been reported from over 50 countries including India. Leaf rust disease was first noticed in India in 1869 and caused severe damage to the flourishing plantations during late 18'h to early 19'h century, especially at lower altitudes. The importance of leaf rust increased again when this disease invaded the Latin American countries during 1970-1985, as these countries are responsible for production of over 80% of arabica coffee.
Coffee rust attacks mostly leaves and very rarely the young branches. Initially pale yellow circular spots of 2-4 mm diameter appear on the lower surface of the leaves, which later turn to orange yellow powdery mass of uredospores. These lesions enlarge and become more irregular as they coalesce with the adjacent spots. With ageing, the central portion of the lesion becomes brownish and later necrotic, whereas the middle zone of the lesion continues to sporulate. Severe infection can cause heavy defoliation and dieback of branches.
Rust affects the berry yield by reducing photosynthetic efficiency due to defoliation and loss of vigor of the plants. In severely affected areas the pathogen may cause foliage loss upto 50% and berries upto 70%.
Causal organism and life cycle
In nature, the fungus produces only uredinia, telial and basidial stages. But perpetuation of the fungus in nature occurs only by uredospores. The teliospores are produced only during unfavorable conditions and germinate in situ under favorable conditions by producing basidiopsores, which are functionless on coffee. Alternate hosts have not been reported so far. Pycnial and aecial stages of the fungus have not been noticed either natural or under controlled conditions. The fungus has an ability to exist in different physiological forms called "races". The pioneer work of Mayne in India to report four races during 1930's paved way for carrying out extensive studies on physiological specialization of this fungus at CIFC, Portugal during 1950's. So far CIFC differentiated rust samples from 37 countries and 45 different races were identified. Among these 33 races have been reported from India alone.
Favorable factors and disease progress
Wet weather during May to November with wind, intermittent rain and sun shine, mist or rain during dry weather from November to March, thin or no over head shade are .the favorable factors for the disease development.· Mayne described the disease development in four phases viz.
- Period of extension (April to September),
- Period of intensification (September to November),
- Period of defoliation (December to January)
- Period of inactivity (January to March).
Under suitable conditions, the disease makes its appearance after blossom showers during March -April. The foliage present at that time is .mostly of previous season and the fungus remains dormant. Such leaves during dry weather alIow the fungus to sporulate and produce fresh uredospores. Simultaneously, vegetative growth of coffee. plants starts after receipt of su I)1mer showers. The disease intensifies further on unprotected younger leaves tiII August and also on new flush developed in the second vegetati ve growth cycle duri ng August -September. The disease reaches its peak during SeptemberNovember and results in severe defoliation. Different views exist on the mode of dispersal ofuredospores. Rayner, Baker and Kranz and Bawden et al believed in air dispersal and wind transport of uredospores, while Nutman et al, Bock and Rajasab et al supported the splash dispersal. Burdekin was of the opinion that both wind and water playa part in the spore dispersal.
Integrated approaches for rust management
a) Growing resistant/tolerant arabica cultivars
Twelve cultivars of arabica evolved at CCRI and released for commercial cultivation showed greater degree of field tolerance. All the selections showed medium to high level of vertical resistance even though attack of specific rust races noticed. Yield of all the 12 cultivars showed a remarkable increase over that of susceptible Kents.
Catimor/Cauvery which showed high degree of resistance at the time of release during 1985, became highly susceptible to rust incidence on account of evolution of seven new races which necessitated to adopt regular chemical control.
Selection 5B(Devamachy X S.333) and Sln.9(Hy_brido de Timor X Tafarikela) showed high degree of vertical durable rust resistance even after 20 years of their release for the commercial cultivation.
b) Cultural method of control
Cultural method of control aimed at either reducing the inoculum potential to reduce the damage or encouraging the healthy growth of the host plant. The importance of this method lies on the basic factor that no additional expenditure or efforts areput except adjustment in carrying aut timely operations. Before the advent of the chemicals or resistant varieties, the cultural methods formed important means of keeping the infection level in check. Some of the cultural operations generally carried out in coffee plantations, which help to minimize the rust incidence are discussed below;
Shade plays a vital role in maintaining the eco-system and required microclimate for coffee plantations. Two tier mixed optimum shade canopy with recommended shade trees not only helps to check the biannual yielding pattern to some extent but also avoids over bearing, which is responsible for predisposing of the bushes for severe rust infection. Further, excessive thinning of shade leads to change in soil fertility, micro flora and fauna, which may directly affect the normal growth of the coffee bushes, thereby predispose the coffee plants for attack of leaf rust disease.
ii) Handling and pruning
Pruning during the dry period after the harvest helps to reduce the initial inoculum of coffee rust pathogen as it generally survives in the form of mycelia and uredospores on the older coffee leaves which were infected during the last season. Further, the disseminated uredospores as a consequence of pruning cannot infect the healthy leaves due to unfavorable climatic condition prevailing in that period.
Handling of the bushes prior to the imposition of pre and post monsoon sprays wiII help in effective coverage of the fungicides especially on the foliage.
c) Biological control
Verticilium hemileiaelVerticilium lacanii, a polyphagous species with wide occurrence in the tropical and sub tropical countries generally parasitises insect mites, bugs, plant rusts and powdery mildew fungi. They also live as saprophytes either in the soil or plant debris. Despite wide host range, some adoption seems to occur in this hyper parasite. Verticilium hemileiae has been recorded as hyperparasite of coffee leaf rust in many countries viz., India, Ethiopia, Brazil, Colombia, Ivory Coast, Zaire etc. However, higher field incidence of this parasite was noticed in India Venezuela, Colombia especially during the rainy season under high humid conditions. Under favorable conditions, this hyperparasite completely covers rust uredospores and lesion with white mycelial mass. Consequent of parasitism, uredospores are killed causing necrosis of the rust lesions. Under high humid conditions, this may go up to 70 %, which indicates its potential to reduce the rust inoculum under field conditions. However, slight reduction in the atmospheric humidity ceases the activity of hyperparasite completely. To utilize this hyper parasite as a biocontrol agent for rust management, in-depth studies are under way.
d) Chemical control
i) Bordeaux mixture:
Copper based fungicides especially freshly prepared Bordeaux mixture has been found effecti ve and economical for the control of leaf rust under field conditions. Based on the field trial results, minimum two rounds of 0.5% Bordeaux mixture application has been recommended,
one during May-June before the onset of monsoon and the other during September -October after the South- West monsoon rain recedes. Alkaline Bordeaux mixture shows beneficial effects such as good foliage retention; slow weathering of copper from spray deposits, prolonged persistence, protection and increased yield. Bordeaux mixture being prophylactic in action, time of spray and coverage is most important. Once the infection takes place, Bordeaux mixture has no effect on the pathogen established inside the leaves. Further, the redistribution of the fungicide is very limited and it has to be placed at the site of infection i.e., underside of the foliage. The spray droplet should be small and distributed uniformly on all the leaf surface area available for infection. Small droplets decreased runoff and increase the contact surface on the foliage
ii) Systemic fungicides:
Since 1974, a number of systemic fungicides have been tested. Carboxin, pyracarbolid, triadimefon, propiconazole, hexaconazole, and epoxyconazole have shown curative and eradicant effect on rust pathogen. Both triadimefon (0.02%) and hexaconazole (0.01 % a.i) have been recommended and widely used by the growers in recent years. These systemic fungicides are highly expensive when compared to Bordeaux mixture. Hence these fungicides may be utilized for rust management either as spot application during August September or on highly susceptible cultivars as blanket spray during the post monsoon period of Sept-Oct.
Sudhakar S. Bhat, Mycologist, Coffee Research Sub Station, Chettalli - 571 248, Kodagu Dist.