Advances in Variable Rate Technology Application in Potato in The Netherlands

Precision agriculture is a farming management concept based on observing, measuring and responding to inter- and intra-field variability in crops. In this paper, we focus on responding to intra-field variability in potato crops and analyse variable rate applications (VRAs). We made an overview of potential VRAs in potato crop management in The Netherlands. We identified 13 potential VRAs in potato, ranging from soil tillage to planting to crop care to selective harvest. We ranked them on availability of ‘proof of concept’ and on-farm test results. For five VRAs, we found test results allowing to make a cost-benefit assessment. These five VRAs were as follows: planting, soil herbicide weed control, N side dress, late blight control and haulm killing. They use one of two types of spatial data: soil maps or biomass index maps. Data on costs and savings of the VRAs showed that the investments in VRAs will pay off under practical conditions in The Netherlands. Savings on pesticide use and N-fertilizer use with the VRAs were on average about 25%, which benefits the environment too. We foresee a slow but gradual adoption of VRAs in potato production. More VRAs will become available given ongoing R&D. The perspectives of VRAs in potatoes are discussed.     

Response of black rot in cabbage to spatial distribution of inoculum

Disease progress of black rot in cabbage crop was studied over three years in field plots to compare the effects of uni-focal and multi-focal inoculum applied in equal amounts per plot. Disease progress (plant incidence and leaf incidence) was plotted over time, three dimensional maps were made, and disease aggregation was studied by means of geostatistics, black-black counts and Moran's I statistic. Black rot progress was primarily due to focus expansion. Secondary foci may appear at short distances from the initial focus but they usually merge with the expanding initial focus. Anisotropy occurred occasionally but was of minor importance. Disease proceeds faster in plots with multi-focal inoculation than in those with uni-focal inoculation. Probably, serious epidemics in Dutch cabbage fields originate from large numbers of foci.     

Survival and Extinction of Xanthomonas campestris pv. campestris in Soil

Carry-over of inoculum of X.c. pv. campestris in the soil from one cropping season to the next was studied in field experiments over three years. These studies were supported by laboratory and greenhouse experiments on quantitative assessment of bacteria by bioassay using the Most Probable Number technique, and on recovery rates of bacteria from the soil. The mean recovery rate from artificially infested soil was 58%. Extinction of X.c. pv. campestris in soil infested with infected plant debris proceeded exponentially and extinction rates depended on temperature, as did the decomposition of plant debris. In replicated field plots, over three years, infection foci of black rot disease were established. At harvest time, all plants were chopped and resulting plant debris was rotovated into the soil. The resulting soil infestation was sampled and showed clear infestation foci reflecting the original infection foci of the crop. These infestation foci decreased with time and disappeared after the winter. Follow-up crops remained virtually uninfected. The results show that in The Netherlands good crop and soil management impedes survival of inoculum from one year to the next, so that cabbage can be grown continuously. Polyetic carry-over of inoculum by debris in the soil can be avoided in The Netherlands.     

Growth of Xanthomonas campestris pv. campestris populations at constant and variable temperatures

Quantitative data were collected to describe the relation between temperature and growth of the cabbage black rot pathogen,Xanthomonas campestris pv.campestris (Xcc). Relative growth rates derived from experiments at constant temperatures were used in dynamic simulation of bacterial population development. The relative growth rates were adequate to simulate growth ofXcc populations at constant temperatures but overestimated growth of populations at variable temperatures. This finding gives rise to the hypothesis, that under field conditions, disease development is slower than is expected on the basis of growth parameters obtained from studies with constant temperatures.     

Growth of Xanthomonas campestris pv. campestris populations at constant and variable temperatures

Quantitative data were collected to describe the relation between temperature and growth of the cabbage black rot pathogen,Xanthomonas campestris pv.campestris (Xcc). Relative growth rates derived from experiments at constant temperatures were used in dynamic simulation of bacterial population development. The relative growth rates were adequate to simulate growth ofXcc populations at constant temperatures but overestimated growth of populations at variable temperatures. This finding gives rise to the hypothesis, that under field conditions, disease development is slower than is expected on the basis of growth parameters obtained from studies with constant temperatures.     

Spatio-temporal development of black rot (X. campestris pv. campestris) in cabbage in relation to initial inoculum levels in field plots in The Netherlands

The effects of initial inoculum levels on spatio-temporal development in black rot of cabbage from artificial sources, one per plot, were evaluated in replicated field experiments. The results support the hypothesis that black rot is a polycyclic disease. Its development was inoculum-dependent because the progress rate of epidemics and spatial spread were both positively correlated with the strength of the source. Fast disease development was related to the number of rain days. The spread of black rot, associated with the primary gradient, at first resulted from allo-infection, later followed by a phase during which allo-infection was complemented by auto-infection. Three-dimensional maps of disease severity showed the dominance of the primary focus. Maximum distance of black rot symptoms from the source of the focus was limited to a few metres so that damage to cabbage by focal inoculum was limited to the plants close to the source. Spatio-temporal development and initial inoculum were related. High inoculum levels in point sources resulted in faster outward spread of black rot, and differences between low and high levels were generally significant. Under the conditions of the experiments, performed during three relatively dry seasons, a single source of infection measuring 0.5 × 0.5 m was not capable of spreading disease over all the plants in a plot of 6.5 × 6.5 m. The results imply that severe disease where whole crops are infected, as observed regularly in The Netherlands, can only originate from a large number of small foci per field.     

Possible Mechanisms Influencing the Dynamics of Rhizoctonia Disease of Tulips

looseness-1In two observation fields, where six sites were artificially infested with Rhizoctonia solani AG 2-t, bare patches developed. These patches did not re-occur at the site of infestation in three successive years. In fields with and without artificial infestation, natural infection of tulip bulbs by Rhizoctonia spp. occurred. The spatial distribution of infected tulip bulbs was visualised in maps after kriging. The influence of sampling intensity was evaluated by stepwise reduction obtained in the observed data set of the first year. Omnidirectional semivariogram characteristics did not change when sampling intensity was reduced down to 10%. The average maximum prediction error was minimised at sampling intensities varying from 7% to 25%. Naturally occurring bare patches slowly vanished during successive cropping of flower bulbs and did not re-appear in the fourth growing season. A high frequency of isolation of R. solani AG 2-t in one field (Lisse-2) in the fourth consecutive crop did not result in bare patches in that year. It is hypothesised that a reduction in aggressiveness may account for this observation. In contrast, bulb rot due to Rhizoctonia spp. increased during the observation period. R. solani AG 5 isolates were seldom isolated before the bulbs flowered, but were the dominant isolate from bulbs at harvest. In a growth chamber experiment, it was demonstrated that AG 5 did not account for replacement of AG 2-t. However, it was demonstrated that competition may partially explain replacement of AG 2-t isolates during the growing season. At 18 °C, but not at 9 °C, an AG 4 isolate prevented AG 2-t colonising and infecting iris bulbs when both isolates were introduced together to soil. Rhizoctonia populations develop in relation to soil temperature and plant development. It is hypothesised that a temporal niche differentiation may be one of the mechanisms affecting the dynamics of rhizoctonia bare patch of tulips.     

Measuring field resistance of cabbage cultivars to black rot

Summary Disease progress and gradient curves of black rot on cabbage were evaluated in field plots of the cultivars Bartolo, Erdeno, Perfect Ball, and Roxy in The Netherlands during 1991 and 1992. Plots were inoculated by single sources in the centre of each plot. Individual plants were examined for disease incidence and severity. Disease progress was described by the Gompertz model. The overall measure of absolute rate (disease progress rate r multiplied with maximum disease intensity K) was used to compare cultivar effects on disease progress. Disease gradients were described by the negative exponential model. The percentile distance (distance from the source at which disease intensity reached 1% of the empirical maximum disease intensity) was used to compare cultivar effects on disease spread. Disease severity is more sensitive than disease incidence to calculate the disease progress and spread of black rot. Measures of progress and gradient were about equally effective to screen cultivars for field resistance to black rot. Perfect Ball was the most susceptible, Erdeno and Bartolo were intermediate and Roxy was the most resistant for incidence and severity measures. Increased levels of field resistance reduced the development of black rot in time and in space. Field resistance of black rot is thought to be composed of several mechanisms. Microplots provide a good instrument for the assessment of small differences in field resistance, expressed equally well in disease progress as in disease gradient curves.