Nemaplot hyperspectral data analysis and population modellingEvaluation reinvented


Modelling the population dynamics of Heterodera schachtii

Damage beet cyst nematode
Field symptoms of H. schachtii
Growth reduction by H. schachtii
Heterodera schachtii Cysts, eggs, Juveniles
Sugar beet canopies with BCN

Climate chamber experiments and parameter estimations

Climate chamber experiments were conducted to get just a rough imagination of the parameter range of the BCN model. The experiments were run under constant temperatures from 12°C to 30°C. Young sugar beet seedling have been inoculated with a H. schachtii suspension, relating to a Pi value of ~1500 E&J/100 ml soil. The crops were examined in predetermined intervals, root biomass and nematode individuals per stage have been measured. The duration of each experiment varied from 70 days at 12°C to 20 day at 28°C. With exception of the 30°C trial new cysts were found in all cases. For all users of Degree Days (DD8), a generation needed a temperature sum of 400°C at 12°C up to 560°C at 24°C.
Multiple objectives were set up with these experiments, first to determine the development rates of the nematode stages with respect to temperature and parameter estimation of the related temperature response functions, and second the response the sugar beet to constant temperatures, and third, how the host-parasite interaction problem varies with temperature.

Nematode stage development rates
Fig.1: Development rates H.schachtii stages at different constant temperatures

The first part is straight forward: plotting the development rates (=1/time in days) over time. The fitted temperature response function are used to calculate the transition probabilities of the Leslie model. The development rate of the host is also depicted in fig. 1. As the inoculum has been a suspension, no function is available for the important stage J2. But the required temperature response function can be fitted to literature data (1)

Slightly more difficult is an interpretation of the crop growth observations with and without nematodes. Despite high Pi values, just a few individuals have been found in the plant for temperatures from 26°C to 30°C. The 18°C experiment did show a general poor growth of the sugar beet seedlings and has been excluded from the analysis. Countable numbers of individuals have been found in the 12°C, 21°C and 24°C trials only. Estimating the temperature response function of sugar beet fine root growth have been straight forward (fig 3).

sugar beet development rate at 12°C
Fig. 2a: fitted growth rate of sugar beet roots at 12°C with & without H. schachtii
Sugar beet development rate at 18°C
Fig. 2b: fitted growth rate of sugar beet roots at 21°C with & without H. schachtii
Sugar beet development rate at 24°C
Fig. 2c: fitted growth rate of sugar beet roots at 24°C with & without H. schachtii

Fine root growth at 21°C has been strongly decreased with nematodes (fig. 2b), also at 12°C and 24°C, but with significant lower rates (fig.6 a, c). Constructing a ratio of no of cysts/dry weight root in mg, we find the following numbers: 0.65 at 12°C and 24°C and 4 at 21°C., i.e. at the end of the 21°C trial a small root system contained plenty of cysts. The unexpected and not stringent result requires some more thoughts.

Development rate comparison of J2 and sugar beet
Fig. 3: Comparison of the development rates of J2 Stages (blue line) and sugar beet fine roots (red line) and measured data

It might be helpful to compare the temperature response functions of both the host and the penetrating J2 stages (Oostenbrink, 1967 (1))in a first step. The system are not synchronized. Different temperature ranges are supporting the host dynamics, others the nematode. This could be one hypothesis for the results of the climate chamber experiments. The fine root system expands a slightly faster than the J2 hatch at 12°C, that means, the fine root system (behind the root tips) is a little bit advanced, therefore available for J2 penetration. The root system holds more and more nematode, until a critical density is exceeded. A classical, evolutionary host - parasite relationship, which guarantee the long time survival and reproduction of H. schachtii without the complete destruction of the host. The 12°C temperature scheme represents in fact to standard condition of the spring situation of both parts. Exposing both "components" to a ~24°C scheme, again the root growth rate is much faster than the hatching rate, growth is not retarded by J2 stages, even as more nematode penetrates the fiber roots, the crop can tolerate much higher population pressures. The root is simply growing away. Certainly a upper boundary exists, but apparently has not been reached in this experiment. Completely different is the situation in the 21°C trial. The hatching rate of the J2 exceeds the growth rate of the host, the crop is flooded with penetrating J2, the root systems remains small, the nematode is limiting itself, some sort of density limitation. It appears the crop has a type of "capacity" or tolerance level, which varies with plant age and growth and is therefore controlled by temperature. The host - parasite interaction is not only impact related to H. schachtii, but also controlled by the possibility of the root system to expand.

What are the implication of these hypotheses? Experiments and tests with and about  H. schachtii are performed mainly in glasshouse experiments at warmish conditions. That is reasonable in the first instance, as the test duration is seriously shorter, one generation of the nematode is completed within 3 to 4 weeks. But, for example, testing the pathgenicity of the nematode or the resistance level of a new variety, the following might happen: the pathogenicity of the nematode is overestimated in the temperature range of 18 bis 22°C, at 24°C under estimated, for resistance/tolerance attributes vice versa. Ergo, the results and conclusion from experiments performed at warmish condition might be doubtful.

(1)OOSTENBRINK,M. (1967), Studies on the emergence of encysted Heterodera Larvae, Neded.,Rijks fac.Landbouw.Jour. 32, 503-539.

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