May 2014. A postdoctoral scholar from India is wanting to correlate severity of foliar and incidence of soilborne diseases with yield, and requests assistance with anyone who is interested in this subject.
A member gave a long response using his experience with groundnuts as an example:
I struggled some years trying to predict yield of groundnut from biomass data, influenced by selected harmful organisms.
My data included number of plant organs (leaves, pods, etc.), leafarea and biomass (of roots, stems, petioles, leaves, pods), harvested at 14-day interval in field trials where plots were protected with or without insecticide and/or fungicide. Leaves were divided over healthy leaves, leafspot infested, leafhopper burn, and thrips damaged and rust was quantifified on a 0-10 scale. Armyworm, leafminer, blister beetle and others were not always present (and if so, often in low numbers) and were controlled or omitted from the datasets. Soilborne diseases were not of importance in the 35 trials I analyzed.
I found that number and longevity of the leaves was the best indicator of yield levels. If you keep the foliage green, especially by controling the leafspot diseases, the optimal moment of harvest (when 75% of developed pods are mature) was up to three weeks later than in the affected plants. In these last weeks, about 500 kg of pods per ha is added per week. Yields could be up to 5000 kg/ha, while unprotected yield was usually around 1500 (this was about the same for Suriname/Guyana, Andhra Pradesh and Indonesia). The data were fed in crop growth models from different origin (DSSAT, PNUTMOD, WOFOST and others) but the results were erratic. After looking in detail how these models operate, I found that a fair number of the growth parameters used by these models were taken from soyabean under the assumption that the parameters for groundnut would be about the same. This proved false. As I did not have the time nor opportunity to establish the values of these critical parameters myself, I had to postpone the exercise until the value of these parameters are established for groundnut. If anybody knows whether these data have become available for groundnut, I would be grateful to see them.
In my opinion, use of disease severity data is difficult for estimation of productivity. They can of course be used to document the effect of pest management treatments, but a direct link to yield seems impossible at the moment. However, something can be done using the data if linked to the physiology of the plant:
leafspots (Mycosphaerella/Cercospora) reduce photosynthesis of the infested leaves and cause premature shedding at a later stage; rust (Puccinia) burns off leaves, these are lost soon after becoming heavily infested, often only when leafspots are less severe; thrips (Scirtothrips, Sericothrips, Thrips palmi and other) and leafhoppers (Empoasca, Amrasca, etc) reduce photosynthesis and may skim off some of the nutrients for growth and sustainance of the plant; defoliators (Spodoptera, GLM) reduce leaf area and are of influence if leaf area index drops below 2.
Soil borne diseases often cause death of infested plants and the effect on yield depends on the time of death:
- if early, surrounding plants may compensate the loss to varying extend (runners compensate better than bunch types);
- if late, the loss is closely related to the relative area that becomes bare.