By G. K. Mahapatro and K. Chandrashekar
Virus diseases are a serious threat to tomato cultivation, and virtually in absence of any recognized or recommended antiviral products, management strategies rely chiefly on host plant resistance, hygienic practices; or on eradication of affected crops and chemical control of insect vectors. Ever-increasing international travel and trade of seeds and fruits have enhanced the risk manifold in Maharashtra, particularly in summer tomatoes. The changing climate, the spread of newly introduced viruses, and/or the associated sap-sucking insect vectors in relevant areas are the potential threats.
Covid continues to its dreaded second wave surge across the country, including Maharashtra; unfortunately, the tomato virus complex is back again wreaking havoc in Satara, Ahmednagar, and Pune districts. Farmers alleged that their lucrative red, juicy tomatoes have turned into yellow, spongy, and plastic textured fruits, despite following recommended guidelines of pest management provided both by public and private sectors. Last year, at the same time, multiple viruses infected tomatoes in Maharashtra (Down To Earth, 12-June & 1-July 2020).
Maharashtra farmers start growing summer tomato in the month of February, and the first harvest commences by April and continues to cater to the market demand till July. Last year, farmers complained about the problem of early ripening and substantial yield loss due to virus disease in the irrigated belt of Pune, Satara, Ahmednagar, and Nashik districts and the hyperactive media house (both social and print) made it more viral. This problem propped up again this year too. Cultivation of Rabi- Summer tomato crop is the most preferred in these irrigated areas, often dominated by hybrids (>90%) that meet the required fruit quality standard to the export demand.
During last year (May 2020) IARI Regional Station, Pune; received a few virus-infested tomato samples from Kolwadi and Alandi areas of Pune District and detected viruses viz., Cucumber Mosaic Virus (CMV), Groundnut Bud-Necrosis Virus (GBNV), Tomato Mosaic Virus (ToMV), Pepper Mottle Virus (PMoV), and Potato Virus Y (PVY). This year too, we have collected samples during second fortnight of May from Narayangaon, Junnar district, where about 1,000 acres of area affected, the lab-diagnostics and detection is under process. A locale market (Junnar, Narayangaon) survey revealed the resultant of virus infection , tomato price crashed as low as to Rs. 2 to 3 per kg. Over recent years, several virus diseases, including Tomato leaf curl virus (ToLCV; genus Begomovirus), Pepino mosaic virus (PepMV; genus Potexvirus), and have emerged in greenhouse and open field tomato crops and are presently impacting fresh-market tomato production in diverse geographic areas worldwide. Tomato Yellow Leaf Curl Virus (TYLCV) affects tomato cultivation in several countries in the world is known to reduce the lifespan and fecundity of its insect vector, Bemisia tabaci, and the virus is transmitted trans-ovarian (passing next generation through eggs), but not through seeds and soil. In India, two major strains of ToLCV (New Delhi and Bangalore) exists causing leaf curling disease in tomato. Viruses may undergo alteration to inhabit a newly introduced niche. The Cucumber Mosaic Virus (CMV) is also an increasing threat trend in tomatoes. Seed companies are consistently maintaining that CMV it is not seed-borne in tomato. However, at least one research report says otherwise in the case of pepper (Journal of Virological Methods, 2010, 163(2): 234-237). Further confirmation is need of the hour.
Troublemaker insect-vectors
Sap sucking insects aphids, whiteflies, and thrips are the agents of spreading many viruses to target crops. Intensified frequent insecticidal treatments, often unilateral and injudicious pesticide utilization as advised by pesticide dealers and agro-consultants (vicious contract between the two). International trade and travel have compounded the vector problem, with the emergence of new species, strains/biotypes/genetic groups.
A multitude of the virus transmitted by aphids are in a non-persistent manner (virus stays in stylets (mouthparts) for a short span in aphids, wherein the aphids dont need to colonize on the crop, their short probe/feeding is sufficient to spread the disease. This makes most of the insecticidal treatments ineffective. Although private companies conduct training for tomato farmers in affected areas, their major objective is maximizing the business little attention given to proper disease management. It is high time, farmers recognize the need for adopting suitable disease management practices in this endemic region. Planting tall barrier crops like maize around the tomato crop is very help full in reducing aphid transmitted viruses like CMV. The aphid vectors landing on windbreak/barrier plants loose their virus load from mouthparts before they move on t the main tomato crop. IARI Regional Station, Pune as well as IIHR-Bengaluru advocates, thus the barrier crop.
Scientists of IARI, Pune station during previous years, surveyed the Pune, Junnar, and Nashik areas, and witnessed few solanaceous crop (sweet and hot pepper) fields showing seasonal replacement of Bemisia tabaci with a different, somewhat smaller whitefly, Aleurothrixus trachoides. Solanum whitefly A. trachoides, reported (Journal of Biosciences, 45(1): 2020) as an emerging vector for begomovirus vector, in solanaceous veggies (tomato, potato and bell pepper) that was considered a non-virus vector by European and Mediterranean Plant Protection Organization (EPPO) reports. Similarly, in the case of the other two virus vectors aphids and thrips, it is important to detect and understand the probable change in their pestilence status, seasonal occurrence, species-spectrum and vector-virus interrelationships. The peak population of aphid is seen in this region during December-January and reduced population from March onwards till September (Indian Journal of Horticulture 72 (3), 423-425:2015), While among thrips Thrips palmi is the most predominant thrips species on tomato (Journal of Insect Science, 28 (2): 159-167, 2015) in onion belt of Nashik and Pune with their peak population seen during September to December. These vectors may be extending their peak period of occurrence to coincide with rabi-summer tomato crop. An in-depth investigation in detail, into all these aspects, is the need of the hour. Very recently, we also reported the prevalence of an exotic whitefly Paraleyrodes minei infesting several fruit tree crops in the state (3-Biotech, 11(264): 2021). Certainly, climate change might be causing shifting of pest-spectrum and their seasonal population dynamics that demand researchers due to concentration and concerted endeavors.
Once infested, there is no cure for virus diseases. Prevention is the best policy than cure. Few relevant points are to be pondered in planning a Good Virus Management Practice (GVMP) earlier also (Down to Earth, 12-June 2020). We emphasize on (1) Procuring quality seeds from reputed sources; adopt resistant/tolerant varieties if available. Unfortunately, the recent cases of multiple virus invasions make the task more difficult to get resistant/tolerant stock. (2) Procure good quality seedlings grown in insect-proof net nursery, from reputed vendors. (3) Avoid indiscriminate use of insecticides, growth regulator applications, in large numbers as suggested by pesticide/seed companies. Remember, there is no recognized anti-viral pesticide for virus control, but many unauthentic products available in the market with huge costs claiming to be antivirals. (4) Many of the aphid-borne viruses, are difficult to control by insecticides.
The critical points singled out, is the vicious contract between plant protection consultants for vegetable farmers of Maharashtra with pesticide houses. Quite often they advocate unnecessary pesticides like fungicides and plant growth regulators/tonics, for even virus diseases. Though they know very well, no anti-viral agent does exist for viruses, the much aggressive marketing strategy by pesticide houses, enforces such situations in the field. A glimpse of our earlier report (Down to Earth 12-June & 1-July 2020) implies how the farmers are being exploited by getting advisories for using fungicides and other products, unnecessarily for viruses. Not to name any particular company, nonetheless big MNCs in the business of agrochemicals are resorted to aggressive sale strategy, suggesting even 12-13 applications, in a short duration tomato crop (120-130 days). Such strategies must be regulated. Farmers must return to their local varieties, with Good Agricultural Practices (GAP), for at least 2-3 seasons in endemic areas.
Dr. G. K. Mahapatro is Head and Principal Scientist and Dr. K. Chandrashekar, is the Principal Scientist (specialization: Insect-vectors), serving at ICAR-IARI Regional Station, Pune, Maharastra.
Email: gagan_gk@rediffmail.com