December 2004. An article in the Post Courier Thursday December 9 2004 on bacterial wilt in the highlands of PNG set the stage for a discussion of this disease on potatoes, there and elsewhere. Farmers in Waghi, Western Highlands Province expressed concern over the loss of potato and tomato crops.
It was pointed out that bacterial wilt (Ralstonia solanacearum) of potato, tomato and related solanaceous crops has been present in the highlands of Papua New Guinea for many years. It was reported in Shaw, DE (1984) Microorganisms in Papua New Guinea. The severity was greater 30 years ago than it is today due to generally better understanding of control.
The two factors having most impact on the improved management of the disease have been the use of certified seed (produced within PNG) and advice to farmers that solanaceous crops should not be replanted in the same ground. From a practical point of view, 5 years between crops seems to work.
Bacterial wilt, Vanuatu
Bacterial witl is severe, even though the “seed” is imported from Australia. A farmer on Efate reported that In the wet year of 2000, most of the crop was lost and potato has not been grown since.
Bacterial wilt, Uganda
CABI has a program in Uganda and the Global Plant Clinic sent the following: Bacterial wilt of potato and other solanaceous crops is a very difficult disease to stop once introduced to a field and can quickly spread within a region. For potato, spread is primarily through the distribution and planting of infected tubers, but soil and water are also important routes. Once present, alternate hosts, including crops and weeds, and the good soil survival properties of the bacterium ensures persistence. Bacterial wilt is also a highly diverse pathogen, with races and biovar described that relate to various and in some instances very wide host distributions. Resistant varieties are much talked about, but remain elusive.
In developed nations, where success can currently be claimed in the control of bacterial wilt in potato, the emphasis for control has been placed on use of seed that has been robustly tested for freeness from bacterial wilt [certified], the interception of infected seed lots and the removal from potato production of land with a history of infection [length of restriction depends on the purpose of use for the potato, e.g., consumption or seed]. By example, certification standards for bacterial diseases in potato in the UK and the EU are very rigorous and are also recognised as being very exacting in implementation. The main feature that has underpinned and accounts for the success of the UK/EU example has been the ability to trace seed lots between generations such that when a outbreak is observed the parentage and full progeny of the initial stocks can be taken out of production and a decision taken on whether fields have been exposed to infection with the consequence of restrictions of use. Such practices are only possible in a very structured industry.
In less structured farming systems, informal seed [seed without certification] is often shared between farmers and planted. This seed quickly looses its history, comes with no assurances of health and when bacterial wilt is known in the region will, in time, result in the disease entering a farmer’s land with long-term impact. However, whereas it is easy to recommend planting certified seed with zero tolerance levels for bacterial wilt it is also necessary to understand why informal seed systems are commonplace and why it proves so hard for many nations to realise a zero tolerance level. In essence, a good certification system for potato is a highly demanding proposition that is dependent on a highly formalised system of production, a robust certification system heavy in technical and human resource needs and a high level of political will!
In Uganda, CABI is currently, with the support of the UK Government DFID, working with a group of local farmers to self-manage multiplication of national programme seed through an improved informal system that does give assurances of history and health. Currently, whereas this production is not certified and has not achieved zero tolerance for bacterial wilt, the seed produced is recognised as better than that previously available in the area and is commanding a higher market price. Already the ambition of these farmers is constrained by the limited availability of national programme seed entering the system. In this work we are partnering a local NGO [AT Uganda] and national agriculture research organisations [NARO, MAAIF].
Bacterial wilt on tomato, India (Tamil Nadu)
Local varieties, which have sour tasting fruit, do not seem to be susceptible to bacterial wilt, although yields are woefully low! High yielding varieties especially the imported ones seem more susceptible.
If spotted early enough, a part of the crop can be saved. A sudden drooping of leaves over 2-3 days or less, with whole plants appearing dark and wet, were typical symptoms. Such plants and the ones in the immediate vicinity are immediately uprooted, and bleach sprinkled on the spot. This, along with reduced irrigation, certainly slowed the spread of the wilt.
The wilt tended to spread radially (and very rapidly), giving the appearance of circular patches of sick plants within a healthy field. These patches had heavier soil or poor drainage. In treating larger patches, bleach helped, but the contamination of soil with chlorine became a question, although chlorine is mobile in soil and is said to be leached out quickly. Rotation of infected areas with non-solanaceous crop and/or a dry fallow after a deep ploughing gave satisfactory results.