Mr. Neil van Rij

Crop Protection
Cedara
KZN Department of Agriculture and Environmental Affairs
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Late blight: an enemy of potato producers for centuries, caused by Phytophthora infestans Mont. de Bary.  Control of the disease is through genetic resistance or the use of fungicides.  Genetic resistance is the most economical option.  However, for various market-related reasons producers need to grow cultivars that are late blight susceptible.  For these cultivars, fungicides remain the essential primary defence.

Fungicides are generally applied as a preventative measure (contact fungicide, action on multi site) so as to avoid disease symptoms.  If this is done correctly, yield loss to disease can be as low as 1-2%.  It can, however, depending on the season’s climatic conditions, be as high as 60-70%.  In these circumstances producers would apply the more expensive “curative” fungicides (systemic, single action site).  The use of fungicides as a curative measure is strongly discouraged as it can, and has, led to the appearance of fungi resistant to the specific fungicide.

For the pathogen to infect the host plant and grow, certain conditions are required.  There must be a plant, a pathogen and, finally, conditions suitable for the pathogen to infect and cause disease.

Fig 1.  Disease triangle

Thus, if the conditions are not suitable for the pathogen no disease can occur and this is where the potential to save money or increase the efficiency of fungicides can be seen.  If the producer were to spray only when the possibility for late blight development was certain, then theoretically late blight would not occur.

Timing the late blight fungicide applications can be accomplished by a number of models that have been published (>17 different models).  These all depend on the availability of quality climatic data.  It is possible to predict when fungicide spays are needed and what type of fungicide needs to be applied.  Trials have shown significant yield improvements and increased profit margins using these systems.  Some are better then others and there may be a usage cost.

The use of models to time fungicide applictions for late blight is often met with the question. But how is it practical?  Practically, if a producer has 100 hectares of potatoes then the chance of applying fungicide over the entire area in one day is almost nil, unless the application is by air or the  producer has many ground sprayers and drivers.  However, it is also true that no producer would really be able to plant 100 hectares in one day.  The technology is available but the ability to benefit from it may be lacking.  Fungicide applications have to be prioritised according to planting date, weather conditions, soil conditions and, if different cultivars are grown, the specific cultivar resistance.

The KZN Department of Agriculture and Environmental Affairs currently has a simple system in operation, based on modified Dutch rules.  These were modified by Mr Brian Young (a plant pathologist at Cedara from 1956 to 1994)  in the 70's, and are tried and tested.  The rules predict when infection conditions have been met and thus the time to start applying preventative fungicides.

The rules are as follows:

• Max temperature <24°C for two days
• Min temperature >10°C for the same two days
• Min humidity 70% or higher at 14:00 on each of the two or more days.

If these conditions are met, this is defined as a favourable period.  After two favourable periods have accumulated, a warning is issued by email to farmers to prepare their spraying equipment.  After three favourable periods a warning is issued to start preventative fungicide applications. 

The information has been linked up with the capabilities of the Internet and a Global Information System (GIS) database.  On the preventive fungicide spray map, green dots represent weather stations that have not met the “Young Rules” criteria, orange squares have met the criteria and red triangles indicate disease is present.

Fig 2.  The “Young Rules” map and late blight incidence map from the 2005 season.

Once the “Young Rules” have indicated the need for a preventive fungicide programme, the focus is switched to the second (incidence) map.  It is used to time the inclusion of the first systemic fungicides in the preventive programme.  For the incidence map, areas that have had late blight outbreaks confirmed are coloured red.  If these red areas are close (<40 km) to the grower’s farm then systemic fungicides are included in the preventive fungicide spray programme.

The map is on the Departmental Web page and available for anyone with Internet access.  Alternatively, an e-mailed text message is sent to anyone interested.  The map is updated on the report of confirmed late blight outbreaks.

The advantages of the map are multiple:

• The map is an aid for deciding when to start systemic fungicide applications
• Reduces unnecessary systemic fungicide applications
• Helps prevent fungicide resistance buildup
• Aids advisers in spray decisions
• Allows the industry to know at a click the current state of late blight occurrence in KZN