Flier, W.G.

Kessel, G.J.T.

Schepers, H.T.A.M.

Spits, H.G.

Wander, J.G.N.

English

Wageningen Yield 

  211
Eighth Workshop of an European Network for development of an 
Integrated Control Strategy of potato late blight 
Jersey, England/France, 31st of March – 4th of April 2004 
Exploitation of cultivar resistance using reduced fungicide dose rates, the 
Wageningen UR approach 
 
G.J.T. KESSEL1, W.G. FLIER1, H.G. SPITS2, J.G.N. WANDER2 AND 
H.T.A.M.  SCHEPERS2 
 
1 Plant Research International, P.O. Box 16, 6700 AA Wageningen, The Netherlands 
2 Applied Plant Research (PPO), P.O. Box 430, 8200 AK Lelystad, The Netherlands 
 
Key words: Phytophthora infestans, decision support systems, potato late blight, fluazinam 
 
Introduction 
In 2004, the umbrella plan Phytophthora, a new initiative on potato late blight control in the 
Netherlands, was launched. Within the umbrella plan Phytophthora the Dutch grower 
organisation LTO, the potato industry, the potato trade and Wageningen UR co-operate to 
achieve the common goal of a 75 % reduction of the environmental burden due to late blight 
control within 10 years. Within Wageningen – UR, all P. infestans related research is co-
ordinated and focussed on this goal. The Dutch Ministry of Agriculture, Nature and Food 
Quality is funding the Wageningen UR Research of the umbrella plan.  
One of the aims of the umbrella plan is to better exploit the possibilities of (more) resistant 
cultivars within a potato late blight (PLB) control strategy to reduce the chemical input when 
possible. The objective of this paper is to outline the approach developed by Wageningen UR 
to be able to reliably reduce fungicide dose rates while maintaining adequate PLB control.. 
 
Concepts 
Possibilities to better exploit cultivar resistance in potato late blight (PLB) control include 
application of fungicides in reduced dose rates and application of longer spray intervals. In 
both cases, growers rely on a reduced level of chemical protection (at least for some period of 
time) which must be “supplemented” by cultivar resistance to achieve the required level of 
protection.  
PPO-Special Report no. 10 (2004), 211-214  
  212 
Protective control strategy 
Current PLB control strategies aim to protect potato crops against infection. For this 
purpose, protective fungicides are applied prior to periods conducive to P. infestans infection. 
When application of a protectant has been delayed due to e.g. bad weather, a curative 
fungicide can be employed up to approximately two days after the potential infection event. 
When this curative treatment cannot be applied, growers can wait and spray an eradicant in 
case of infection or resume with a protectant when infection has not taken place. 
 
Disease pressure 
Disease pressure is a rather loosely defined concept describing the infection risk as e.g. low, 
moderate or high. Estimates for disease pressure are based on an observer’s opinion on local 
weather and influx of inoculum or they are derived from local historical weather data and a 
weather forecast. A quantitative estimate of the infection risk of a crop could help to refine 
control measures. Quantification of the actual infection risk to a crop would have to include 
estimates on the (predicted) influx of airborne inoculum, host resistance and (predicted) 
chemical protection level in combination with a local weather forecast. Influx of inoculum: 
Ideally, estimates for influx of inoculum should be based on accurate information on 
available sources and local weather so that source strength, release, survival of sporangia 
during atmospheric transport and deposition can be estimated. Information on local P. 
infestans sources will however never be complete. To avoid unnecessary risks to the crop, 
information on local sources could be replaced by the assumption that sources are available.  
 
Cultivar resistance 
For the purpose of this paper, cultivar resistance may be separated into vertical (absolute) 
resistance and horizontal (partial) resistance. In the absence of compatible P. infestans, vertical 
resistance provides absolute protection against infection and, at least theoretically, does not 
require extra chemical protection. Unfortunately, virulence against many or all R- genes or 
available R- gene combinations is present in most European P. infestans populations. In the 
presence of a (partially) virulent P. infestans population, R-gene containing cultivars can 
become infected when a compatible isolate “lands” in the crop. Thus, when the P. infestans 
population is partially compatible, presence of (an) R-gene(s) may bring about a delay in first 
infection. 
  213
 
Table 1. Components of resistance constituting the level of partial resistance of potato cultivars 
Component of resistance Symbol Unit Description 
Infection Efficiency IE [-] Fraction of successfully infecting spores 
Latent Period LP [day] Time span between infection and production of the 
first spore 
Lesion Growth Rate LGR [mm day-1] Radial growth rate of a lesion 
Sporulation Rate SR mm-2 day-1] Number of sporangia produced per mm2 of leaf per 
day 
Infectious Period IP [days] Time span during which potato tissue remains 
infective 
 
 
Partial resistance provides an intermediate level of resistance against PLB. Usually, four or 
five components of resistance are recognised that together constitute a cultivars partial 
resistance level. These components relate directly to the P. infestans infection cycle and are 
given in Table 1. Partially resistant cultivars are prone to infection but the higher the level of 
partial resistance, the slower the epidemic. 
In practice, potato cultivars combine different levels of vertical resistance and partial 
resistance but for the purpose of a protective control strategy in the presence of compatible 
isolates the Infection Efficiency (IE) is the only component of resistance from which an 
infection risk can be derived. 
 
Infection risk 
Theoretically, the infection risk of a crop at any moment in time can be derived from the 
partial level of resistance (IE), the (remaining) level of chemical protection, the estimated 
influx of sporangia and the weather conditions in the near future (e.g. expected leaf wetness 
duration). During the growing season this estimated infection risk has to be kept below the 
maximum acceptable infection risk. 
The maximum acceptable risk can be derived from e.g. the infection risk of a fully susceptible 
cultivar just before it has to be sprayed again with a protectant. 
 
To be able to implement this strategy, a series of field experiments was carried out, reported 
upon elsewhere in this volume) in which IE was estimated for the 30 most important 
cultivars in the Netherlands. Additionally, in a second series of field experiments each of 
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these cultivars was sprayed with 6 Shirlan dose rates (0%, 20%, 40%, 60%, 80% and 100% of 
the recommended dose rate) to derive the minimum dose rate required. Both the minimum 
dose rate required and the IE  are used to derive the dose rate for a set of 5 cultivars, 
representing a range for IE, used in two validation field experiments under practical 
conditions.  
 
The infection risk is influenced by cultivar, (expected) weather, atmospheric influx of 
sporangia, the remaining level of chemical protection and the dose rate applied. From these 
factors, the dose rate applied can be adapted to achieve the required protection level. 
 
 
  


Exploitation of cultivar resistance using reduced fungicide dose rates, the Wageningen UR approach

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Exploitation of cultivar resistance using reduced fungicide dose rates, the Wageningen UR approach

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