By and large, plant protection measures depend heavily on chemicals, many of which have been demonstrated to be hazardous to the environment besides producing resistant/tolerant strains of the pathogens. Some of the diseases, which were of minor significance, have become devastating due to iatrogenic effects. Therefore, an integrated disease management approach is perhaps the only strategy to keep apple diseases below the economic threshold level, without endangering the environment. Some of the management strategies are discussed below:
Strict Observation of Quarantine Practices:
It is a well-established fact that the apple industry has suffered largely due to the violation of quarantine practices against scabs. The disease which, was restricted to only Kashmir Valley till 1973, has spread over all the apple-growing regions of the country. The spread of apple scab to the NEH region including Sikkim can particularly be attributed to the large-scale transportation of planting material from Himachal Pradesh during 1979-1985, despite the scab epidemic in the state. The growers, and the consuming public. and the nation as a whole is bound to pay a heavy price for this indiscretion. There are a number of devastating apple diseases still not reported in India e.g. fire blight (Erwinia amylovora (Burrill) Bergey et al.), and rust (Gymnosporangium juniperi Virginiana Schwein). If the quarantine legislation is not implemented, the future of the apple industry will be at stake.ย
Procurement of Certified Planting Materials:
Planting Material
Procurement of planting material from certified sources and identification of certified mother plants for scion wood are the only ways to minimize the spread and losses caused by various viral and mycoplasma diseases. Infrastructure for indexing against these pathogens, and rapid multiplication through micro-propagation techniques have to be strengthened to meet the increasing demands of the growers. The introduction of most soilborne diseases such as root rots, collar rot, crown gall, and hairy roots could be easily avoided by implementing nursery certification legislation strictly.
Scientific Orchard Management Practices:
Orchard Management
Scientific orchard management practices aimed at providing adequate phytosanitary conditions in the orchard are summarized below.
- Pruning of diseased twigs, shoots, or branches, collection, and destruction of leaf litter and pruning waste, spot application of chemicals on canker-infected tissues, pit treatment/nursery beds with chemicals or fire stove, and 5% urea spray at pre-leaf fall stage are essential to minimize the primary inoculum, which delays the initiation of most of the diseases.
- Pre-planting dip treatment of grafted plants in broad-spectrum fungicides checks the introduction of soil and sapling-borne diseases to the new orchards.
- Proper sterilization of grafting/pruning tools and field implements helps in reducing the spread of viruses and soilborne diseases.
- Careful pruning, high grafting/budding (>30 cm), wound dressing and trunk painting with a protective paste before the onset of rains, and avoidance of tillage-intensive intercrops in the orchard floor helps in decrease the pre-disposition of apple plants to canker and soilborne diseases.
- General practices of watershed management and construction of check-dams ensure water retention in the orchard, which improves the general health of the trees. Construction of diversion channels on the upper boundary of the orchard slope helps in maintaining proper drainage in low-lying areas, which discourages the incidence of root rots.
- Pruning of diseased twigs, shoots, or branches, collection, and destruction of leaf litter and pruning waste, spot application of chemicals on canker-infected tissues, pit treatment/nursery beds with chemicals or fire stove, and 5% urea spray at pre-leaf fall stage are essential to minimize the primary inoculum, which delays the initiation of most of the diseases.
Build-up Host Endurance Through Balanced Nutrition:
Plant Nutrition
Plant nutrition has a profound impact on the resistance /susceptibility of apple trees to various diseases. Except for nitrogen, information on the influence of other macro/micronutrients on plant endurance to diseases is lacking. Imbalanced fertilization with NPK has been reported to increase the susceptibility to diseases, particularly apple scabs (Kumar and Gupta, 1986). There is a need to find out a balanced schedule of nutrition with respect to the host’s reaction to diseases and pests. In this context, the role of mycorrhizae association needs to be explored.
Forecasting of Disease Outbreaks: Integrated Management of Apple Diseases
Spry On Tree
Disease prediction is important for enhancing the efficacy of chemical control, and for making it economical besides preventing environmental pollution by curtailing unnecessary pesticide applications. In India, scab fore-warning services are being provided in the three major apple-producing states (Gupta and Thakur, 1992). Information on disease development is communicated to the growers. This enables them to undertake timely sprays. reschedule spray programs according to the prevalence of infection periods. Fore-warning systems need to be set up in different apple groves to monitor the micro-climate precisely and improve the precision of predictive systems.
Judicious use of Broad Spectrum Fungicides:
Fungicide Spry on Tree
A number of fungicides have been recommended for the control of different apple diseases. Excessive use of these chemicals is not only a threat to the environment but there are other adverse effects on non-target organisms in the phyllosphere and rhizosphere. Excessive use also leads to the development of resistant strains of pathogens, and contamination of edible fruits causes health hazards.
Benzimidazoles are the most important systemic fungicides currently in use for the control of apple diseases. Resistant strains of scab pathogen (V. inaequalis) against benomyl and dodine have been reported from the USA, Netherlands, Australia, and Japan (Gilpatrick and Blowers, 1974; Wicks, 1974; Jones and Ehret, 1976). In India, the emergence of benzimidazole-tolerant strains has been reported from Kashmir Valley (Qasba and Shah, 1987; Qasba et al, 1988).
Soil drenching has been recommended for the control of soilborne diseases in India. Systemic fungicides specific to Pythiaceous fungi such as metalaxyl, efosite-Al have been recommended for this purpose (Ellis et al., 1982; Gupta and Mir, 1983a; Rana and Gupta, 1984b). Deleterious effects of soil-drenching fungicides on mycorrhizae have been reported, depriving the trees of Mycorrhizae’s beneficial effects. Carbendazim is not effective against Pythiaceous fungi, and the use of carbendazim 0.1% in the rhizosphere for the control of white root rot needs re-evaluation.
Continuous use of systemic fungicides should be discouraged. Fungicide rotation should be followed to reduce the chances of the emergence of fungicide-resistant strains. Broad-spectrum fungicides, with low residues in fruits, should become a part of integrated management strategies for all diseases of apple rather than evolving control measures for a particular disease.
Development of Biological Control Methods:
Biological Control Methods
The introduction of antagonists for the biological inhibition of apple diseases has been reported. Some of the antagonists like Trichoderma viride Pers. ex Pers. S.F. Gray., Enterobacter aerogenes (Kruse.), Hornaeche and Edwards. and Bacillus subtilis Ehrenberg Cohn) have given encouraging results against root rots (Orlikowski and Schmidle, 1985; Gupta and Utkheda.1986). Colonization by Athelia bombacina Pers. and Chaetomium globosum Kze. suppresses the formation of pseudothecia in the overwintering apple leaves (Heye and Andrews, 1983; Cullen et al.,1984). The introduction of T. viride and solarization by polyethylene sheet trapping of the soil improves the control of white root rot. The introduction of Chaetomium and its metabolites produced in the culture is effective in suppressing the development of scabs and powdery mildew. Treatment of mildewed apple seedlings with conidia of hyperparasite Ampelomyces quisqualis Ces. significantly reduces the disease (Haseli, 1988). Root extracts of Rumex obtusifolius have shown significant control of powdery mildew (Seifried et al., 1988).
The role of Vesicular-Arbuscular Mycorrhizae (VAM) and ectotrophic mycorrhizae in providing resistance against soilborne pathogens is well established, but so far no work has been carried out in this area in India. There is a need to investigate the impact of the introduction of antagonists. VAM fungi, and organic amendments to keep the population of the pathogenic fungi below threshold levels.
Resistance Breeding:
Host plant
Host plant resistance forms an essential component of the integrated management strategy for apple diseases. Disease-resistant cultivars are the most economic means of disease control besides ensuring a healthy environment. However, resistance breeding in fruit crops is time-consuming and needs long-term planning. There is a continuous fear of the appearance of virulent strains of pathogens and other iatrogenic effects of minor diseases attaining devastating status. In apple, the resistance breeding program against major diseases is in progress in several countries.
Scab:
Scab Disease.
The plant resistance breeding program was started in 1948. The hybridization between M. domestica X M. floribunda 821 resulted in the first five scab-resistant selections in 1967. Apart from the Vf gene, monogenic (Va, Vr, Vb, Vbj, and Vm) and polygenic (M. baccata, M. sargenti, M. sieboldii, M. toringo, and old European varieties) sources of resistance have been used. More than 28 cultivars have been released so far. Of these, 11 have been evaluated in Himachal Pradesh. Emra and Red Free are performing better in mid and low-hills (Sharma et al., 1988).
Powdery Mildew:
Powdery Mildew of Apple fruit.
Few genetic sources of resistance or immunity to powdery mildew are known viz. M. robusta (PLI), M. zumi (PL2), MIS (Mildew Immune Seedling) obtained from seeds of Starking Delicious, M. hupehensis cv White Angel, Ergi Red, David, and MA 8. Scab-resistant varieties; Liberty, Sir Prize, and Prima exhibit resistance to powdery mildew. Evaluation of native indigenous material revealed that M. baccata types collected from Shillong and Kashmir possess higher levels of resistance than M. zumi (Sharma and Kishore, 1992).
Collar Root Rot: Integrated Management of Apple Diseases
Collar Rot Disease of apple fruit.
High levels of resistance for Phytophthora cactorum are present in James Grieve, Laxton Superb, and Melba. The resistance of Northern Spy appears to be under single gene (Pc) control. The hybridization of Northern Spy resulted in a series of resistant rootstocks such as MM 106, MM 109, and MM 111. Evaluation of apple rootstocks against collar rot has shown that M 2, M 4, MM 105, MM 113, and MM 114 are resistant under field conditions (Gupta and Mir, 1983b).
White Root Rot:
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White Root Rot of Apple Fruit.
No significant success in host plant resistance has been achieved against this disease. M. floribunda and M. toringoides show considerable resistance to D. necatrix (Anderson, 1956). Cydonia oblonga, Malus prunifolia var Ringo Asami, and M. purpuria are resistant to low disease pressure (Sharma and Kishore, 1993).
The objective of apple resistance breeding is to obtain cultivars with multiple resistance against major diseases and to utilize polygenic resistance to offset the emergence of more virulent pathogens. Liberty is resistant to major diseases and can be grown without fungicidal protection. Unfortunately, it is severely affected by Alternaria leaf blight in Himachal Pradesh. latrogenic effects open a debate on various aspects of resistance breeding and the necessity of an integrated approach for disease management rather than depending on any one method of disease control alone. Progress in this direction has already been initiated in the case of apple scab management.
Reference:
Plant Protection and Environment
P. Sharma and D. K. Kishore
Indian Agricultural Research Institute, Regional Station (Horticulture) Amartara Cottage, Cart Road, Shimla 171004, Himachal Pradesh, India.
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