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Effects of fertilization on pests and diseases Y. HOFMEESTER Research Station for Arable Farming and Field Production of Vegetables, Lelystad, The Netherlands Accepted 9 January 1992 Abstract
The aim of integrated farming systems research is to develop sustainable fanning systems from an economic, technical and environmental point of view. Strategies for crop rotation, crop protection and fertilization are briefly discussed in relation to potato production. The effects of different levels of fertilization on the incidence of some pests and diseases of potatoes are discussed in more detail, based on a literature study. Some results of potato production on the three farms for integrated arable fanning in the Netherlands are also presepted.
Additional keywords: integrated fanning systems, potato crop rotation, crop protection, fertilization, integrated arable fanning Introduction
Integrated farming systems research in the Netherlands was started in 1979 at the Nagele experimental farm. Results obtained on this farm initiated two new experiments in 1986 (North-East) and 1989 (South-East) to develop integrated farming systems for the specific regional conditions. The overall strategy for integrated farming systems is described by Vereijken and Royle (1989). It aims at the shift in emphasis from greater production to cost reduction and improvement of quality both of products and production methods. Healthy crop rotation is the basis for integrated arable farming, while strategies for crop protection and fertilization differ considerably from the conventional approach. With regard to the symposium 'Advances in potato crop protection', this is elaborated in relation to the potato crop.
Crop rotation.
Crop rotation has different objectives: (1) to maintain the soil fertility, (2) to minimize the need for pesticides, and (3) to achieve an acceptable economic result. This means a recommended maximum frequency of 1:4 for potatoes to avoid serious problems with soil borne pests and diseases in particular. Cereals are preferable as a preceding crop because their large rooting system creates a good soil structure for the potato. The following crop should be easy to handle with potato volunteer plants, for instance vegetables or sugar beets.
Fertilization.
The objectives of an integrated fertilization strategy are both economic and ecological: an acceptable profit and a minimum of unwanted emissions. This may be achieved by considering the following guidelines (Vereijken, 1990): (1) maintain soil fertility at a level which is not too low for adequate yields and not too high to produce high quality products; (2) dosing and application of minerals should be aimed at maximum 257
uptake by the crops and minimum emissions; and (3) fertilizers must be substituted wherever possible by organic manure. This means in most cases the application of organic manure before the potato crop. Potatoes are very sensitive to a good soil structure which is improved by organic manure; it also has a positive effect on the yield of potatoes. The manure should preferably be applied in spring by injection, or cultivation should take place immediately after application. The demand for P and K can be covered, while nitrogen is added by means of additional artificial fertilizers in split application, based on soil or leaf analysis during the growing season. If 60% of the required amount is applied before planting, a more efficient use of nitrogen is obtained (Van Loon and Houwing, 1989).
Crop protection.
To minimize the input of chemicals to control weeds, pests and diseases and maintain an acceptable level of control, the strategy is based on: (1) prevention, (2) determination of the need for control, and (3) control, by biological, mechanical and, if necessary, chemical means. As far as pests and diseases of potatoes are concerned this means using (partially) resistant or more tolerant cultivars in the case of the most important ones such as potato cyst nematodes, virus diseases and Phytophthora infestans. Lowering the nitrogen level should also contribute to a decrease in disease incidence. If control of certain diseases or pests is necessary, this should be done on the base of damage thresholds, which are available for Rhizoctonia solani, potato cyst nematodes and aphids and viruses (Vereijken and Van Loon, 1991). When biological methods are not available, chemicals are chosen based on efficacy, selectivity, toxicity for man and animal, persistence, mobility and cost. A moderate (nitrogen)-fertilization is one of the methods of preventing severe problems with pests and diseases. In most cases a lower nitrogen rate means lower fertilizer costs, but this should not result in a less economical yield. According to Van Loon and Vos (1990), potatoes only consume 50% of the amount of nitrogen generally given. Lowering the nitrogen rate by 25% of that required for maximum tuber yield results in the net returns being reduced by only 2% and also causes less loss of nitrate into the environment. It has the additional advantage of improving several quality parameters of potatoes, such as reduced second growth, a higher dry matter content - which is important for the industrial processing of potatoes - less tuber deformation and a lower nitrogen content (Van Loon, 1989). So, lowering the nitrogen rate has different positive effects on potato production. According to Vereijken and Van Loon (1991) there are several major pests and diseases of potatoes, which are controlled chemically and therefore substantially contribute to the total cost of potato production. Thus lowering the fertilization rate could result in an extra cost and pesticide input reduction. In this article some of the main pests and diseases are discussed, based on a literature survey. Zitzman and May (1988) stated that in many cases soil N-content has positive effects on insect fitness, presumably through its intermediate effect on plant N-content, although a variety of plant physiological responses to N-fertilization could influence insect feeding. On the other hand N is also a component of a variety of plant antifeedants and toxins and may be present as non-utilizable nitrates. This might be the same for fungi and/or bacteria and therefore all different kind of relationships might be expected.
Phytophthora. Pehl and Sturm (1962) stated that the potato crop becomes more susceptible to P. infestans at the moment of demolition of proteins in the plant, which is more or less at the time of tuber formation and foliage closing between rows. They carried out a 3258
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Table 1. Weight percentages of infected tubers (mean of 15 sites). Fertilizer treatment (kg h g l) N-0 N-50 N-100 N-150
8.5 9.1 * 9.9** 10.0"*
P-0 P-39 P-78 P-117
10.5 9.4* 9.2" 8.4**
*, ** = Significant differences within columns (P < 0.001 for N, P < 0.05 for P) From : Herlihy (1970)
year field experiment with different nitrogen rates and a constant P and K rate of 80 and 120 kg ha-~ respectively. Nitrogen rates varied from 0 to 120 kg N ha-a, with graduations of 40 kg. The higher the nitrogen rate, the lower the incidence of P. infestans. They conclude that the potato crop is less susceptible to P. infestans when the N-supply is at a good level because of the high protein N level in the leaves. On the other hand, Herlihy (1970) found that the higher the N-fertilization, the higher the weight percentages of infected tubers (mean of fifteen sites), as presented in Table 1. The attack by P. infestans is facilitated by the increased canopy of foliage at higher nitrogen rates. This created favorable conditions for proliferation of P. infestans on the foliage, resulting in an increase in transfer of spores to the root zone, and therefore tuber infection. However, it may also reflect the fact that tuber maturity is most advanced in potatoes grown at low nitrogen levels. Lenticels and tubers become less susceptible to penetration by the fungus as the tuber matures. This was also mentioned by Scholte (personal communication) in relation to
R. solani. Carnegie and Colhoun (1983) mention a linear relationship between fertilization rate of N, P and K and lesion size and growth rate of P. infestans. This was only found when fertilizers were added at the planting stage, but not later. The older the leaves the higher the growth rate and the bigger the lesion size within a fertilizer treatment. All their experiments were conducted in glasshouses with artificial inoculation with zoospores. Weindlmayr (1965) also found an increase in susceptibility to P. infestans with increasing nitrogen rates (under greenhouse conditions). Increasing rates of phosphor at a moderate nitrogen level decreased sporangial development of P. infestans, which corresponds to the findings of Herlihy (1970) (Table 1).
Virus diseases. Virus diseases are of great importance for both seed and ware potato production. Hunnius (1967) carried out a number of pot- and field experiments to examine the effect of increasing nitrogen rates on infection with viruses and on mature plant resistance to PVYN N. He assumes three mechanisms for the effect of nitrogen on the incidence of PVY N. Through the higher N inputs, the mature plant resistance is delayed. The growing period is extended and so more time is available for the virus to multiply after infection and to move to daughter tubers. Thirdly, the multiplication rate of aphids is higher and so a larger infective aphid population is present. In pot experiments over a period of 3 years (1964-1966), the percentages of infected tubers were as shown in Table 2. Nitrogen rates increased by 1 gram per pot. N~ is a treatment with optimum rates of N, P205 and K20 for potatoes (ratio 1:1.5:2), whereas in the other treatments the nutrients are not in correct proportion. There is a significant decrease in N 1 in a number of infected tubers (6 days after Neth. J. Pl. Path. 98 (1992) Supplement 2
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Table 2. Percentage of tubers infected with potato virus Y at increasing nitrogen rates and different days of infection.
NO N1 N2 N3
infection 8 days before flowering
14 days later
mean
51.7 50.3 64.3 71.6
37.4 28.3 50.7 37.9
44.5 39.3 57.3 55.6
From: Hunnius (1967) flowering) which might be explained by the mature plant resistance. The difference between N 2 and N 3 (mean) is very little, but this is due to the disproportional mineral amounts. This also explains the higher percentage of infected tubers at N O, which represents disproportional fertilization. When infection occurred 8 days before flowering, the percentages of infected tubers were rather high at all levels of nitrogen, although there was an increase with increasing nitrogen rates. In field experiments (120 kg P205 and 180 kg K20 ha -l) with different cultivars, Hunnius only found in one single year a clear relationship between the highest nitrogen level and infection with potato-leafroll virus (differences in N: 60 and 120 kg ha-l). This relationship was most evident in the moderately to slightly susceptible cultivars, 5.9% leafroll at 60 kg N ha q and 8.2% at 120 kg N h ~ 1. Differences with the resistant cultivars were only 0.8% and 1.4% respectively. During the other two years, no differences between N-treatments were found. Hunnius (1967) concluded that the conditions at early infection mainly determine the abundance of infections. When the aphids are controlled at an early stage and the potato crop develops quickly, a higher N input is justified. Harrewijn (1983) proved that the population development of both Myzus persicae and Macrosiphum euphorbiae decreased with an increase of potassium. Both the adapted fertilization (with four times more potassium than the basic fertilization) and the mineral oil resulted in the availability of less essential amino acids, which may render the plant less suitable for both aphid species (Table 3). Experiments with artificial diets proved the relationship between physiological conditions of plants and number and development of aphids. This can be influenced by fertilization and cultural measures such as the use of micro-elements, for example lithium (Table 4).
Colorado potato beetle.
Adding compost to potatoes growing on sandy soil greatly decreased the number of potato beetles. This may be explained by higher plant resistance Table 3. Number of pre-alatae per 100 leaves of Myzus persicae. Treatment
23/7
29/7
4/8
11/8
basic fertilization mineral oil adapted fertilization
3 3 7
32 20 19
49 21 25
8 2 5
From: Harrewijn (1983) 260
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Table 4. The effect of a single combined leaf spray with Cu++-and Li+-ionson the proportion ofprealatae (%) of Macrosiphum euphorbiae in a field with potato plant cv. Bintje. Date of application: July 23, 1982. Treatment
22/7
29/7
5/8
11/8
125 kg N ha L 70 kg N h~ 1 Cu++and Li+
12 8 11
19 14 0.2
19 15 0.8
2 1 0
From: Harrewijn (1983) to the beetle under optimal growing conditions (Grussendorf, 1953). In experiments carried out by Schaefffenberg (1968) the potato beetle could choose between unfertilized potatoes and potatoes fertilized with compost. Within 48 hours more than 80% of the beetles moved to the non-fertilized plants. In another experiment where equal amounts of beetle larvae were put onto fertilized and non-fertilized plants, mortality on the fertilized potato plants was 76%, on the non-fertilized ones 47%. On the fertilized plants, leaves were far less consumed, indicating that they were less attractive or contained a 'killing element'. In any case there was a chemical-physiological change in these leaves which might be related to the sugar content. Sugar content is lowered by fertilization and therefore the host plant is in a less favorable physiological condition for the insect, so its mortality increases. Another mechanism is described by Jansson and Smilowitz (1986). Developmental and feeding rates of the colorado potato beetle are temperature dependent. Feeding rates may therefore be accelerated on plants low in nitrogen, because these plants are generally warmer than fertilized ones. In other experiments they found that the developmental and feeding rates of the colorado potato beetle were negatively correlated with the foliar total N in potato, which might correspond to each other.
Black leg. Logan et al. (1987) studied the effect of different rates of N (0-120 kg
ha-l),
P (0-420 kg ha -x) and K (0-240 kg ha-1) on the incidence of black leg and gangrene. None of the levels an any of these minerals affected the incidence of black leg or gangrene except for the case when P was omitted. Then the incidence of black leg increased. The wetter and colder the spring, the greater the incidence of black leg. This is explained by slow plant growth due to low temperatures. The high soil moisture encouraged bacterial multiplication in inoculated tubers. For gangrene there might be a biological factor with a marked effect on the incidence of gangrene, which varies quite considerably from farm to farm.
Potato cyst nematodes.
The effects of N, P and K on the population density of potato cyst nematodes (PCN) are studied in only a few cases. Stelter and Effmert (1987) did a 7-year field experiment with potato monoculture. They had four different treatments: non-fertilized, organic manure (30 ton ha -1), mineral fertilization and 15 ton organic manure combined with 50% mineral fertilizers. The reproduction in the unfertilized plots was lowest, while the treatment with organic manure (100% or 50%) did not differ significantly except for one year. In pot experiments with compost and quartz sand they found no differences in either the number of nematodes or root mass, where nutrition conditions for N, P and K were equal (pot size 170 cm2). They concluded that the effect of organic manure resulting in a higher number of nematodes is caused by a better mineral condition in these treat-
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Table 5. Nutrient and humus percentages of the fields. Fertilization
Nt (%)
P (mg/lO0 g)
K (mg/lO0 g)
Ct (%)
humus (%)
pH
without organic manure mineral fert. org./min. (50]50)
0.08 0.17 0.10 0.15
7.2 22.8 10.8 22.0
14.5 48.0 31.5 44.0
1.06 1.87 1.02 1.67
1.83 3.22 1.75 2.87
6.2 6.2 5.6 6.1
From: Stelter and Effmert (1987) Table 6. Effects of three rates of fertilizer and two cultivars on the final population density of Globodera pallida (eggs g-i soil). Fertilizer
cv. Cara
cv. Pentland Dell
306 791 718 558
53 94 209 101
(t ha -r) 0.5 1.0 1.5 Mean
Least significant ratio (P = 0.05): body of table x 3.16, mean • 2.61 From: Trudgill (1987) ments, as shown in Table 5. Furthermore humus favors the transport of air and water through the soil, producing better root growth conditions. This resulted in higher yields but also stimulated reproduction of PCN. The effect of N, P and K on potato growth and yield of cultivars which differ in their tolerance of damage by PCN was studied by Trudgill (1987). In this experiment, the initial population density was 123 eggs in 1 gram of soil. He found a decrease in number of nematodes in plots receiving the two lower rates of fertilizer when the non-tolerant cultivar 'Pentland Dell' was grown. In contrast the tolerant cultivar 'Cara' consistently increased the final population density, with the largest increase occurring in plots receiving the two higher rates of fertilizers (Table 6). Although there is a difference in tolerant and non-tolerant cultivars, a larger number of nematodes occurred with increasing fertilizer rate, according to Stelter and Effmert (1987).
Verticillium wilt. As Davis et al. (1990) pointed out in their article about verticillium wilt on 'Russet Burbank' potato, rotation and disease resistance are two major components in controlling the wilting disease. As with other diseases, verticillium is also influenced by cultural management, but these cultural relationships may not necessarily be the same in every region and every year. Irrigation is one of these cultural practices, but it seems to be a matter of nitrogen rather than water availability. When N is distributed uniformly (with sprinkler irrigation), less wilting occurs than with furrow irrigation (no uniform distribution of N). In a 3-year experiment the effects of P and N were clearly shown. A split application of nitrogen reduced the number of propagules of Verticillium dahliae in the soil significantly. For phosphor there was a significant decrease in disease incidence at all levels of P (120 and 240 kg haL), although no explanation could be given. 262
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Discussion and conclusions In general the increase in nitrogen - and in some cases the decrease in phosphor - levels resulted in an increase in pest and diseases of potatoes (except for the colorado potato beetle). However, it is difficult to compare the different investigations on this subject because of the different conditions (pot or field experiments), inoculation methods and fertilization levels. The mechanisms involved are different and related to the biology of the pest or disease. Sometimes physiological changes in the plant tissue (proteins, amino acids, sugar content) are responsible for the disease incidence, sometimes plant growth stimulates the pest or disease by creating more feeding sites or better microclimate conditions. In the integrated farming systems research in the Netherlands potatoes are grown according to the guidelines described in this article, which include moderate nitrogen fertilization to minimize problems with pests and diseases. This resulted in a reduction in input of insecticides and nematicides (expressed in kg active ingredients per ha) of 100% (except for the control of aphids in seed potatoes) compared to the conventional systems. For the fungicides the reduction varied from 0 to 60% depending on the location. Almost all fungicide treatments are used to control late blight, indicating the need for varieties resistant to this disease.
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Van Loon, C.D. & Houwing, J.F., 1989. Optimization of nitrogen nutrition of ware potatoes. Publikatie nr. 42, PAGV, Lelystad (in Dutch, with English summary). Van Loon, C.D. & Vos, J., 1990. Ge'l'ntegreerde teelt van aardappelen. Dossier Gewasbescherming 7: I0-12. Vereijken, P., 1990. Integrated Nutrient Management (INM) for arable farms. Schweizerische Landwirtschaftliche Forschung 29: 359-365. Vereijken, P. & Royle, D.J. (Eds), 1989. Current status of integrated arable farming systems research in Western Europe. IOBC/WPRS bulletin XII/5. Vereijken, P. & Van Loon, C.D., 1991. A strategy for integrated low-input potato production. Potato Research 34: 57-66. Weindlmayr, J., 1965. Untersuchungen fiber den Einfluss gesteigerter Stickstoff-, Kali- und Phosphorgaben auf die Phytophthora-Anf~illigkeit yon Kartoffelpflanzen in N~ihr-16sungskultur. Die Bodenkultur 16: 144-168. Zitzman, A. & May, M.L., 1989. Growth, food consumption, and nitrogen and lipid compositions of the Colorado potato beetle, Leptinotarsa decemlineata, (Coleoptera: Chrysomelidae), as a function of the nitrogen supply of its host plant. Journal of Entomological Science 24: 62~69.
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