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Evaluating the effect of common bean mixtures in controlling pests in Uganda

June 6th, 2012

An experimental design to evaluate the effect of common bean (Phaseolus vulgaris) mixtures in controlling the damage caused by Angular leaf Spot and Beanfly in Uganda, a discussion by scientists.

Reacting to a proposal made by students, the scientists involved in the pests and diseases project gave the following suggestions. The students originally proposed to limit the experiment to four varieties.  The scientists recommended increasing the amount of diversity.  They suggested that instead of holding experience that compared mixtures with 1,2,3 and 4 varieties, we organize the experiments with an exponential increase, e.g. 1, 2, 4, and 6 varieties. . In addition, the scientists pointed out that the experiments should consider the level of richness that has been observed earlier on the farms over the last four years of the project so that the experiments reflect what farmers’ current practices are.  The earlier studies showed that the mean richness on farm was less than 3 varieties at household level, but the pool of varieties that the farmers were selecting from at the community level ranged between 12 and 27.   Moreover, as the question was to look at the impact of richness and evenness on disease and pest regulation, it was important to examine more varieties than the on- farm norm grown, to see if the effect of growing more varieties in different mixture experiments had an effect on disease regulation.  It would also enable us know if the benefits are exponential, asymptotical or to draw any kind of curve that informs us on the trend in pests and disease pressure when increasing the number of varieties. They also noted that It would be interesting to see at what level diversity is playing the role perhaps there is a threshold. The scientists made it clear that a balance between asking scientific questions and doing what farmers might find practical can both be accomplished with one careful design. 

The other challenge was about selecting the varieties to use in the mixture trials considering the diverse community pool. The original idea was to use a combination of susceptible and resistant varieties to see the extent to which the susceptible can be protected by the resistance, irrespective of the popularity. However, if whatever comes out of these trials is to be interesting to the farmers, including the popularity of varieties is inevitable; it makes the trials more realistic and practical. Scientists pointed out that although it is important to honour what farmers are doing now, this is a research project and we need to consider current practices in light of what might be gained by moving beyond or changing current practices to improve the overall outcome for farmers (transformative research). Since the main scientific question revolves around the role of diversity in controlling pests and diseases, choosing varieties with greater diversity of resistance to include in the study may result in data that is more meaningful in addressing the hypothesis of increased diversity resulting in less damage. Scientists emphasized including the full range of resistance available in the experiment and include combinations of varieties with differing levels of resistance. This allows you to demonstrate a principal that can be expanded to different situations. For example, you may find that varieties with moderate resistance grown in combination are adequate to limit damage in most situation and that highly resistant varieties are only necessary under some circumstances. Using moderately resistant varieties has the advantage of placing less selection pressure on the pathogen population, which can compromise durability of resistance. Basing on the above, scientists recommended having a balancing act where selection of different combinations is to be based on the varieties’ resistance levels and agronomic qualities especially those that determine their popularity among farmers in addition to including susceptible and resistant checks; since the resistance and popularity levels of varieties are known from earlier studies.

The issue of which combinations to make also came up. Scientists pointed out that If one has 6 varieties to work with, then one has many different combinations of two varieties, fewer of 3, etc., and only one combination of 6.  There is need for a way of deciding which combinations one wants to test, unless there are lots of resources that warrant to testing all possible combinations.  The number of replications for each combination might not have to be equal, depending on one’s analyses, for instance, if all 2-variety combinations are considered as equal when disease is plotted versus richness.  On the other hand, maybe comparisons among the different 2-way or 3-way combinations are what is considered most important; then one needs enough replicates for each to make this distinction.  The other suggestion was to randomly choose combinations (with or without replacement), or to choose a manageable number of varieties and then form all possible combinations among those. One could start with the most popular varieties and choose the best 6, 8, 10, or whatever can be handled and make combinations among these.  If they do not vary in resistance, then there is trouble!  Or, one could compromise and take the 4-5 of the most resistant and 4-5 of the most popular and make mixtures of these in various combinations of  res + pop; 2 res + 1 pop, 1 res + 2 pop, etc. Scientists recommended taking the above approach and working backwards to the number of treatments one can afford but noted that it all depends on which questions are most important. They went further that it’s going to be a compromise in some way where one has to figure out what is being lost or gained with each decision. 

The types of plots and the number of treatments to make could not be left out of the discussion. Scientists noted that square plots are ideal and then disease or fly assessments should be done in the insides of the plots, not near the margins.  Smaller plots of less than 5x5m would allow testing more treatments in the same area but this should not sacrifice the accuracy of the data. What are the physical limitations of the study area? What is the maximum number of plots of the chosen size that can be evaluated in a single location within reason? With answers to these questions, one could determine physical limitations regarding experiment size and number of treatments that can be included with appropriate replication. This will likely limit the number of options that can be considered.  Three replications is minimum for field plot research. The scientists recommended adding another replicate and limiting the treatment numbers to provide greater power in differentiating among treatments.  Scientists made it clear that treated seed is essential as seed borne innoculum could confound varietal results if one variety has more seed infected than the other. If results are already known about alternating rows versus within-row mixtures, then is it necessary to repeat this part of the study. Within-row mixtures will reduce disease more, but it might not be practical for marketing or eating. 

Doing the mixture trials on-farm is a bit tricky as different climatic conditions are conducive to different pests and diseases as the case is in Uganda where highlands are more conducive to Anthracnose and lowlands to Angular Leaf Spot. With that in mind, scientists recommended having the mixture trials carried out in different environments apart from the screen house.