Susan Dimbi, Rhoda Masukwedza and Dahlia Garwe Tobacco Research Board. Harare.

Transcription

1 FAILURE TO DESTROY TOBACCO STALKS ON TIME CAN POTENTIALLY DESTROY THE ZIMBABWEAN TOBACCO INDUSTRY! Susan Dimbi, Rhoda Masukwedza and Dahlia Garwe Tobacco Research Board. Harare. INTRODUCTION For decades discipline and trust, including strict following of the relevant, practical and science-based advice provided by the Tobacco Research Board, were at the core of the tobacco industry in Zimbabwe. These enabled the Zimbabwean tobacco growers to consistently produce and present for sale, a world class crop. In the 1960s, though, tobacco production in the country 1

2 was severely threatened by the aphid-transmitted virus diseases of tobacco [Potato Virus Y (PVY) and Bushytop virus disease] and only research-based solutions which were adopted, implemented and strictly followed by growers, saved the day. More recently, complacency and failure to adhere to the well-established recommended practices, is once again putting the Zimbabwean tobacco industry at risk. In this paper we elucidate why and how the legislation on Tobacco Planting and Stalk Destruction Dates was arrived at. THE APHID-TRANSMITTED VIRUS DISEASE MENACE In the late 1930s and 1940s, PVY and bushytop devastated the Zimbabwean crop in the northern tobacco growing areas. In trials done at the Tobacco Research Board by Legge in 1963, it was shown that there could be as much as 100% yield loss to PVY and bushytop-virus diseases, especially in the late planted tobacco crops. In 1979, Shaw, another TRB researcher, described these diseases as the greatest single threat to tobacco production in Zimbabwe. After years of research elucidating the relationship between the aphid-vector populations, the disease incidences and the severity, a science-based solution was developed and implemented. The secret to managing this menace was simply an Integrated Pest Management Strategy (IPM) anchored on adherence to certain legislated sowing, planting and stalk destruction dates. DECIDING ON THE LEGISLATED TOBACCO PLANTING AND STALK DESTRUCTION DATES The requirement put in place regarding stalk destruction is enshrined in the Plant Pests and Diseases Act (Chapter 19:08) which states that; Tobacco plants (stalks and re-growths) must be destroyed before 15 May and lands other than those being used for the current season s crop should be free of living tobacco plants at all times. 2

3 Additionally, other relevant sections of the Act state that seedbeds should not be sown before 1 June of each year and seedlings should not be transplanted into the fields before 1 September of every year. It further states that all seedbeds must be destroyed as soon as seedlings are no longer required for the current season s crop and no later than 31 December each year. Adherence to these dates ensured that there was no continuous growing of the tobacco crop from season to season, but that there was a dead period created, when there was no readily available food source for the aphid vector. The legislated dates were based on data collected by TRB researchers, which clearly showed that populations of the carriers of virus diseases (the winged aphid) fluctuated greatly throughout the tobacco growing season (Fig 1). The general trend was that populations were lowest at the height of the dry season, due to food scarcity. However, after the onset of the rains and the planting of crop hosts in November, the winged aphid populations built up very rapidly, reaching a peak between mid-december and mid-jan before falling off again under the pressure of predators and parasites and as host plants began to senesce. In June-July some resurgence in the population was observed, probably as the predator parasite populations collapsed, coincident with some redistribution of aphids on dry season host plants. Populations reached their lowest at the height of the dry season in October, attributable to the scarcity of suitable host plants. Consequently, crops planted at a time when aphid populations were high, succumbed to PVY and bushytop virus diseases. Additionally, the continuous availability of a food source (the tobacco crop) naturally maintained and haboured aphid populations from one season to the next, putting the subsequent crop at risk. Based on this information, legislation aimed at preventing the uninterrupted growing of tobacco was enacted. This was done to disrupt the life cycle of winged tobacco aphids and prevent the establishment of the virus diseases they transmit, through the creation 3

4 of a dead period (15 May 15 June) during which there would be no tobacco crops growing in the whole country No. of aphids per square meter leaf area Fig 1 : Seasonal winged aphid populations trends (work done by Shaw in 1967) After implementation of the legislation, there was a breakthrough as aphid-transmitted virus diseases ceased to be a menace. This success was fully attributable to the sincere, individual and collective responsibility taken by all the tobacco growers. For decades thereafter, the Zimbabwean grower faithfully and diligently ensured that this legislation was strictly adhered to. THE RESURGENCE OF APHID-TRANSMITTED VIRUS DISEASES Sadly, in the 2005/6 season, the aphid and the coincident virus disease threat returned. Once again aphids and virus diseases became a huge challenge with 100% yield losses 4

5 being reported on flue-cured tobacco (Fig 2a and 2b). Blame was immediately placed on the development of an insecticide-resistant aphid species. Fig 2: (a) a flue-cured tobacco field showing 100% plant infection by bushytop virus disease and (b) plants infected by both the bushtop virus disease and PVY virus. To establish the veracity of this thinking, numerous trials were promptly set up at Kutsaga. The immediate aim was to evaluate the susceptibility of the tobacco aphid to the then registered aphicides. Additionally, in an attempt to establish if the seasonal winged aphid population trend still held, TRB researchers re-established the Shaw trials. Aphid traps were set up around Kutsaga in the exact spots where Shaw had set up his trials decades earlier and all catches were collected daily, identified and the numbers recorded. The results were both exciting and shocking! It was clearly established that the registered aphicides were still effective and that the previously established winged aphid population trend still held beautifully. However, there was a huge increase in the numbers of winged aphid populations during the January peak. Figure 3 below shows that a mean of winged aphids per square meter leaf area were recorded in the 2006/7 season in comparison to 500 aphids in 1963/64. 5

6 / / Oct Nov Dec Jan Feb Mar Apr May June July Aug Sep Fig 3 : A comparison of the winged aphid catches for 1963/64 and 2006/7 seasons. This revelation clearly showed that contrary to the theories being touted at the time to explain the increase in virus disease incidences, Zimbabwe had not been invaded by a super insecticide-resistant aphid species but that it was mere complacency that had put the tobacco growers in this invidious position. Inadvertently growers had been rearing and nursing cultures of potentially virus-carrying tobacco aphids in their own lands from season to season and with the plentiful and uninterrupted food supply, populations had exploded! While this predicament could be partially explained by a learning curve on the part of new growers on tobacco farms, it was also evident that the discipline that had kept the industry ahead of the pack was no longer there. Many growers even today, are knowingly placing individual gain before the greater good. There have been cases where growers have made decisions to cut costs by not conducting stalk destruction while others believed that they could increase yield and the subsequent returns from 6

7 tobacco farming by sowing tobacco seed in May and planting as early as August, despite the legislated dates. APHID TRANSMITTED VIRUS DISEASES STRIKE AGAIN Various attempts have been made to avert the looming disaster. TRB religiously sends out calendar-based stalk and seedbed destruction reminders to growers annually. These Dear Grower letters clearly advise growers of the aphid trends and the requisite action as well as reminders on when to sow and plant tobacco and when to destroy stalks. Additionally, in partnership with the Department of Specialist Services, numerous awareness campaigns, to educate growers were carried out throughout the tobacco growing districts. The Tobacco Industry and Marketing Board, AGRITEX and other stakeholders have also been tirelessly urging growers to comply with this legislation. However, all these warnings have continued to be ignored by some growers to the extent that sightings of a newly planted crop next to happily growing undestroyed stalks from the previous season are common, while the planting of the irrigated tobacco crop in August by many large scale growers is now an open secret. As a consequence, PVY and other virus diseases have continued to proliferate as the winged tobacco aphid (Fig 4a) simply flies from one crop to the next. In the current season, 2017/18, symptoms of the most damaging PVY strain (the necrotic strain) have been observed on many crops (Fig 4b) and the TRB Plant Clinic has been inundated with samples of infected plants from growers. The bad news is unlike in the past, this disease set in much earlier and affected even the early planted crops. OTHER FACTORS THAT AFFECT DISEASE SPREAD AND INTENSITY Additional factors that exacerbate this problem is the growing of other crops in the same family as tobacco such as potato, tomato and eggplant in the same field, on the same farm or on nearby farms. These crops are such deadly alternative hosts for aphid- 7

8 transmitted virus diseases that in the 1964, legislation was introduced in Zimbabwe to prevent potatoes and tobacco being grown at the same time on the same farm between November and March. This was after PVY infection rates of 40-50% were being reported by growers. The implementation of this piece of legislation worked very well to reduce disease pressure on tobacco for the next seven years until it was repealed in Thus, tobacco growers should as much as is possible minimize the growing of this crop in close proximity to potato. 4a 4b Fig 4: (a) The winged tobacco aphid on tobacco and (b) a tobacco crop infected by the necrotic strain of PVY. THE WAY FORWARD - BACK TO BASICS The resurgence of the aphid-transmitted virus diseases on tobacco and the severity of infection could destroy the tobacco industry as we know it. It was the looming disaster in the form of the potential collapse of the tobacco industry from virus diseases, that got the Tobacco Research Board, the legislators and the whole tobacco industry to get together and put in place a sustainable preventative strategy in the form of tobacco 8

9 planting and stalk destruction legislation in the 1960s. This worked for decades and ensured that the industry thrived. Today the remedy for this menace is as simple as the growers and the rest of the industry Going Back To Basics and adhering to recommended disease preventative measures that have been proved to work. It is everyone s responsibility to ensure the sustainable growing of the crop as the future of tobacco in Zimbabwe is under threat from aphids and the diseases they transmit. SOME ADDITIONAL NOTES ON PVY AND BUSHTOP VIRUS DISEASES: 1. Potato Virus Y (PVY) and bushytop-virus diseases are spread by the winged form of the tobacco aphid (Fig 4a). 2. Once established in a crop, PVY has been shown to be also mechanically transmissible as well. Thus necessary care must be taken to prevent spread. 3. PVY and bushytop virus diseases are NOT transmissible through tobacco seed. 4. Disease control for PVY and bushytop-virus diseases is through prevention of the establishement of the disease by controlling the vector populations and by enabling plants to escape infection through early planting. 5. Once a virus disease has established on a crop, no chemical is effective in curing the affected crop. Ignore legislation at your own peril! 9