Plant disease management in organic farming

Ngangbam Punita / Nongthombam Devachandra *

Brief introduction

Human mankind would always remain grateful to the numerous agricultural researchers and scientists associated with Green Revolution (GR) of the World as well as India. When we talk about GR, the names of Dr Norman E Borlaug and Dr MS Swaminathan pop up in our mind, though there are many more of them to acknowledge.

All of them could successfully drive out ‘hunger’ of the masses by attaining ‘food security’ during 1960s and 1970s. The GR was associated with cultivation of High Yielding Varieties (HYVs) especially of wheat and rice.

However, there has been extensive investigations about the after-effects (rather ill-effects) of the successful adoption of production technologies • to cultivate HYVs on soil and environment. While debate continues, alternative production technologies are popularized. The most prominent being organic farming or organic agriculture and latest one being ‘natural farming’.

Organic agriculture (OA) is a holistic production management system that promotes and enhances wholesome agro-ecosystem health, including biodiversity, biological cycles, and soil biological activity. The concept of organic farming is not pristine although it gained its popularity in the last decade. It was presented to the world during 1940s by Sir Albert Howard, Father of Organic Farming after working along with the Indian farmers in Madhya Pradesh for several years.

Basically, the practice of OA is something that our forefathers had been on practice but was declining during and after GR. It is reported to have practiced in 187 of 196 countries in the world, and 72,30 million hectares of agricultural land are managed organically by at least 3.10 million farmers. On the other hand, India is home to 30% of the total organic producers in the world, which is a huge contribution.

With the increasing global food population and limited land resources under agri-allied sectors, safeguarding food security along with associated ecosystem is quite a challenge for modem agriculture. However, disease management strategies in present organic agriculture scenario needs to be re-evaluated. Mainly because, pests and pathogens are best and effectively managed with ‘agrochemicals’ despite having numerous hazardous consequences.

Challenges Under Organic Farming

i) Inability to manage once epidemic breaks out
ii) Orientation towards single and static management strategy
iii) Risk of faster dissemination of pathogens vis-A-vis globalization
iv) Climate change that results in too less or too much rain, extreme temperatures, natural calamities etc.
v) Difficult to manage foliar diseases under humid conditions
vi) Limited number ofresistant/tolerant varieties
vii) Effects of organic approach are not immediate
viii) Effective implementation and entangled legislative measures under the Destructive Insect Pest Act, 1914.

Sustainable Plant Disease Management Approaches

One of the main objectives of the new approach for plant disease management in OA is to aim for a holistic management strategy rather than one static management practice. It is said that occurrence of a disease depends on the balanced action, reaction, and interaction among five components viz. (a) host (b) pathogen (c) environment (d) time (e) microbiomes and (f) anthropological intervention (whether intentional or unintentional).

The holistic management system of diseases includes two main components: achieving multiple goals (higher yield, efficiency, better quality, and safety) and integrated approaches to understand the evolutionary ecology of host-pathogen interactions.

A. Legislative Approach

The Government of India legislated the Plant Quarantine Order in 2003 under the Destructive Insect Pest Act, 1914 to regulate the import and export of plant materials. The ICAR-NBPGR (National Bureau of Plant Genetic Resources), New Delhi- issues Import Permit and carry out quarantine processing of imported Plant Genetic Resources (PGR) and issues Phyto Sanitary Certificate for PGR meant for export.

As a commendable service, during 2015-2020, ICAR- NBPGR intercepted the imported seed samples to be infected/infested by fungi and bacteria (50%), insects (17%), with weeds (13%), nematodes (13%) and viruses (6%).

B. Cultural Approach

B.1 Choice of Geographical Area For instance, common beans are very susceptible to anthracnose and bacterial blight when grown under humid conditions. Therefore, growing of beans in drier irrigated regions of western USA resulted in the production of plants and seeds totally free from Colleto-trichum lindemuthianum, Xanthomonas phaseoli and Psuedomonaspheseolicola.

B.2 Field Selection: Apple orchards on cleared oak forest land are often infected by collar rot leading to decline in number of trees.

B.3 Disease Escape Varieties Early maturing varieties of pea effectively escapes powdery mildew as the crop gets mature earlier before the environment becomes favourable for pathogen growth.

B.4 Time of Planting : Late sown (end of November to December) chickpea shows little or no incidence of root rot and wilt in North Indian condition.

B.5 Seed and Planting Material Selection : Utilization of poor-quality diseased seeds not only creates an unhealthy plant but also acts as source of inoculum for disease spread.

B.6 Crop Rotation : Unavailability of host exudates for a few years (or cropping seasons) results in starvation of the pathogen. However, this method is not advisable for pathogens that are soil inhabitants that can survive in the soil for quite a long period (sometimes up to 40 years!) through the formation of resting structures (like oospores, sclerotia, chlamydospores etc.) and for pathogens that have a wider host range. Interestingly, it has been reported that pineapple-banana rotation significantly suppresses banana wilt (Fusarium oxysporum) through the secretion of root exudates from pineapple that increases the antagonistic microbial population (microbiome) in the rhizosphere.

B.7 Removal and Destruction of Disease Plants or Organs : Eradication of alternate and collateral hosts that grow during the off season helps in destruction of pathogen that survives on them. Elimination of Pucciniagraministritici (wheat rust) over wintering sites in Fujian province of China by persuading farmers to change their cropping systems from growing winter wheat to potatoes and broad beans had significantly marked the avoidance of major epidemics (wheat stem rust). Bio fumigation with defatted seed meal of Indian mustard (Brassica juncea) can effectively control Rhizoctonia solanimycelial growth up to 61.5%.

B.8 Modification of micro-climate : Post-harvest decay of fruits and vegetables can be reduced by cold storage that provides a modified environment unfavourable for pathogens. Proper training (providing support systems) of vines and removal (pruning) of excess leaves narrow down bunch rot and downy mildew in grapes. Maintaining recommended spacing is imperative in OA as it determines the Relative Humidity (RH) % in the phyllosphere region which in turn is directly proportional to foliar disease development.

C. Vector Control

Applying organic amendments such as application of neem derived products helps in vector control as they act as antifeedants, disturbs the growth and reproduction of the insects. Entomo-pathogenic fungi, Metarhiziu-maniso-pilae strain CQMa421 controls rice plant hoppers under field conditions and shows efficacy upto 60%.

D. Disease Resistance

(i) Increasing host population heterogeneity by intercropping different rice varieties (or genotypes) greatly reduced rice blast incidence. This also applies to other crops.

(ii) R gene resistance is one of the most convenient ways to control plant disease.

However, R gene for every disease and for every plant is difficult or not available in nature; while in other cases, they are tightly linked with certain other genes that are associated with undesirable agronomic traits.

(iii) A new and novel technology to exploit host resistance is through the adoption of genome editing in developing disease resistant varieties. Genome editing helps in the knocking out the susceptible gene in the genome of the host plant, thus manipulating the gene for gene hypothesis and making the plant unrecognizable by the pathogen. Government of India has recently (March, 2022) exempted genome edited products (SDN1 and SDN2; Site Directed Nuclease) free from exogenous introduced DNA from biosafety assessment in pursuance to the Cells Rule 1989.

E. Biological Control

The micro habitat around the plant is home to several microorganisms (microbiome) which are often favorable for plant growth. Enhancing natural control is always preferred over the application of biological control agents in organic farming.

For the naturally occurring strains are more efficient in that given environment than introduced strains. Deployment of rice phyllosphere micro habitants against Magna-portheoryzae resulted fungi-cidal/fungistatic action.

Conclusion

In recent years, there has been a worldwide swing towards eco-friendly methods for protecting crops from pests and pathogens. The challenge today is how to achieve not only ‘food security’ but also ‘food safety’ without compromising ‘nutritional security.’ A common but gravp mistake prevalent among the farming community is taking consideration of managerial aspects only after a disease outbreak.

This outlook may be due to the easy disease control in conventional chemical agriculture as well as extensive propaganda/marketing on effective disease management approaches from agrochemical companies. However, in OA this view needs a reset. Managerial practice should start right from the time when a farmer decides to start cultivation.

It cannot be an individual approach. It needs to be a collective effort involving fellow farmers a contiguous area. All the stakeholders need to have common concern. It is really encouraging that in recent time agri-allied companies/MNCs are orienting themselves towards production, promotion, and propagation of biopesticides, biofertilizers, and biostimulants. They can foresee the demand of these inputs of future agriculture.