Late Blights Forecasting

Forecasting Late Blight: Severity Values

Ideal temperatures for late blight infection are 50-60oF at night and 60-70oF in the day. A relative humidity of 90% in the canopy promotes pathogen development. Spores will die in low humidity. Rain can disperse and trigger spore germination. Late blight has to be treated as a community disease; it can affect everybody. So, everyone needs to do the best job possible to prevent spreading. The most common method to predict when conditions are favorable for the late blight pathogen to develop and spread is by calculating daily 'severity values' from duration periods of high relative humidity (Pavlista and Stevenson, 1995).

The data needed to forecast late blight in a particular field or area are:

1. date of 50 percent emergence of a variety/field [not planting date]

2. maximum and minimum daily temperature

3. duration in hours of high relative humidity

4. rainfall and irrigation: daily water exposure

Late blight forecasting basically warns growers of the potential for the development of the disease and then suggests the rates and scheduling of fungicides. If late blight inoculum is present, it predicts the occurrence of symptoms. This disease develops when there has been an extended period of favorably low average temperatures (below 80ºF) and high relative humidity (80 to 90 percent depending on method of measurement and placement of instrumentation) after emergence. Forecasting is based on the field reaching a severity value threshold.

Once a potato variety’s date of emergence is observed in a field, there are two techniques for obtaining the data needed to calculate severity values. The more accurate technique is for a grower to place a hygrothermograph in the field and collect the temperature and humidity information every week and more often when the threshold is approached or reached. Using this instrument placed at the base of the canopy, high relative humidity is considered to be 90% and above. The other, less accurate but easier and more common, technique is obtaining the information from a weather station. Weather stations are placed outside the field and usually surrounded by mowed grass. Using data from weather stations, high relative humidity is considered to be 80% and above. Weather information can be downloaded through a computer hook-up to a weather station or network. In Nebraska and parts of neighboring States, the High Plains Region Climate Center is a good source (Changnon et al., 1990).

After a grower has obtained the needed data, the severity values can either be calculated directly by a grower using a pencil or paper or can be typed into the “Potato Crop Management” (PCM) program developed by the University of Wisconsin. The PCM program is highly recommended since it has functions other than late blight forecasting, such as early blight forecasting, emergence prediction, irrigation scheduling, growing degree day calculations and Colorado potato beetle predictions.

Most of the information presented is from the PCM manual. The PCM program is based on the “Blitecast” system developed by the Pennsylvania State University (Krause et al, 1975) using data collected and conclusions from studies conducted in western Nebraska (Wallin and Schuster, 1960).


  • “PCM:The Integrated Systems Approach to Potato Crop Management” 1990. University of Wisconsin cooperative Extension Service, Madison, WI.
  • Changnon, S.A., Lamb, P., & Hubbard, K.G. 1990. Regional climate centers: New institutions for climate services and climate-impact research. Bull. Amer. Meteorol. Soc. 71:527-537.
  • Krause, R.A., Massie, L.B. and Hyre, R.A. 1975. BLITECAST: A computerized forecast of potato late blight. Plant Disease Reporter 59:95-98.
  • Pavlista, A.D. and Stevenson, W.R. 1995. Forecasting Late Blight. Univ. Nebraska Coop. Extension Circ. 95-1250.
  • Wallin, J.R. and Schuster, M.L. 1960. Forecasting potato late blight in western Nebraska. Plant Disease Reporter 44:896-900.

Severity Values Determination

Late blight forecasting starts with the emergence date of a particular variety in a field. The determination of whether the threshold of severity values is reached is based on the accumulation of daily severity values from the date of emergence. The date of emergence has been defined as the date when at least 50 percent of the plants have emerged from the ground. A thin green row on top of the hill should be visible at this time. From this date, the daily temperature and relative humidity needs to be recorded hourly.

The severity value is a measure of the interaction between temperature and relative humidity in the presence of the late blight fungus. Table 1 shows how severity values are assigned to specific conditions. If there is no late blight observed in the field nor in a nearby field, then a cumulative severity value total of 18 predicts late blight symptoms are to appear in 7 to 14 days if the inoculum is present and no fungicides are applied.

Table 1: Severity values assigned to different temperatures and durations of high relative humidity, 80 percent relative humidity for weather stations (PDF)

Besides the need to monitor winds coming from regions of the country with late blight, predicting the lag period before late blight occurrence may be related to the speed of crossing the severity value threshold (18). Wet periods that are followed by long dry spells prior to accumulating 18 severity values may allow a low level of development of late blight. When the threshold is crossed, late blight appears sooner, within a week. On the other hand, following a sudden, rapid accumulation of severity values resulting in an immediate crossing of 18, late blight may take three weeks before appearing. It would need that time to undergo its developmental and population process. (observations by Alexander Pavlista, Univ. Nebraska-Lincoln.

Fungicide Recommendations Based on Severity Values -

Once 18 severity values have accumulated from the date of emergence, tracking temperature and relative humidity and assigning severity values should be continued. From this information, appropriate intervals for fungicide applications can be determined. However, one more piece of information needs to be monitored and added now. Recommendations are also based on the amount of water received by the field through rainfall or irrigation (Table 2).

Inches of rain fallen during the past 10 days
Inches of irrigation water applied during the past 10 days

Table 2: Fungicide spray interval recommendations based on severity values calculated from previous week and precipitation the past 10 days (PDF)

Weather Stations

Although field-placed hygrothermographs are the most accurate method for collecting data, data collected from weather stations are being widely used due to convenience. The weakness of these data is that weather stations are located in flat areas surrounded by low cut grasses. There are no obstructions nearby so wind is free to influence the readings and the station is away from wet or irrigated areas. In short, it does not read the microclimate existing in a field of potatoes but the macroclimate of the general area. During the past few years, weather station data have been monitored and compared with hygrothermograph data and late blight appearance. The relative humidity (%RH) threshold for weather station data which has correlated well with field data is 80% RH (not 90% used with hygrothermographs). This has been confirmed throughout Nebraska from 1995 to 1998 and in various locations in Colorado.

The temperatures of interest for late blight forecasting are the maximum and minimum temperatures during a period of high relative humidity (80% RH for weather stations). If the high RH period extends past midnight, record the duration and severity value on the date the period began. Record zero (0) for the subsequent days until a new period begins. The high RH period is considered broken when the RH drops below the high %RH level for at least two hours. Collect rainfall and irrigation data daily.


A recording hygrothermograph basically keeps track of hourly changes in air temperature and relative humidity. Most use a week long chart so the data can be collected weekly. The upper part of the chart gives the temperature record; the bottom relative humidity. They can be either spring-wound or battery driven; the former seems to be more reliable in the field. Their calibration needs to be checked every spring before use; periodic checking during the season is also recommended. Hygrothermographs need to be placed inside a shelter in a field to protect the instrument from rain, hail, wind, etc. A rain gauge, accurate to 0.05 inches, will be needed since once late blight is forecasted, rain data will be needed for fungicide scheduling recommendations.

Hygrothermographs need to be placed in a weather shelter to protect the instrument from adverse weather. A shelter consists of a double-walled roof and perforated or slatted walls to allow free air movement. Lining the inside of the shelter with window screening will keep bees and wasps from nesting inside and keep mice from entering and chewing on the hairs of the RH probe. If you wish to buy a shelter instead of making one, they are available through forestry-supply companies.

For the most accurate readings, a hygrothermograph and its shelter needs to be placed in a part of a field where the likelihood of late blight is the greatest, i.e., the wettest part. Within the first tower of a pivot is recommended. It needs to be in place by the date of emergence. Some suggested locations are:
1. Within the potato canopy, preferably in a low spot of the field.
2. Near the center of a pivot system where irrigation keeps the vines wet.
3. Close to a body of water, e.g., stream, ditch or pond.
4. Near a fence row or woods where air movement is restricted.
5. On the leeward side of windbreaks, e.g., trees, barn.
The point is to place the hygrothermograph where dew appears first in the evening and dries last in the morning. Rain gauges should be placed near the shelter but not so that it is affected by the shelter in the distribution of rain or irrigation. Technically, a hygrothermograph should be placed in every field, but since this is not common, strategic placement is needed.

Hygrothermographs should be cleaned at the season's end, and inspect and replace broken hairs (RH probe). Release the tension on the hairs of the probe with a counter weight. Store the instrument away from rodents and insects.