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Threat and Risk

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Cucurbit Downy Mildew: Threat from a Disease Source and Associated Risk Prediction



Threat refers to disease development factors at or very near the source; Risk refers to the potential spread and development at the source and along the forecast trajectory. Threat is related to weather conditions, disease level, and source area, which determine the source strength (level of spore production). It also includes the potential for transport to other growing areas (favorableness of overhead and nearby sky conditions). Risk refers to the danger of disease development to growing areas along the course of a trajectory (or trajectories, in the case of multiple sources). Table 1 below gives the three levels of source Threat and the associated Risk levels. The association is indicated by a shared boundary between a Threat and a Risk. Note that there is NOT a strict one-to-one relationship between categories of Threat and Risk.



To interpret Table 1, recognize that Threat describes a range of maximum risk values that may be possible for a given spore transport episode. For example, if a source is a Serious Threat, then some area along the forecast trajectory will have a Risk level of at least Strongly Moderate. A source of Moderate Threat will have a maximum Risk level of at least Weakly Moderate, but no more than Strongly Moderate. Conversely, if a source is classified as a Low Threat, that implies that no target area along the forecast trajectory will be at Moderate Risk (or greater).

How are the risk assessments formulated?

We start by assuming that risk of disease development can be divided into three simple categories:

  • Low
  • Moderate
  • High

These three categories are based on factors that are involved in the long-range transport and deposition of spores:

  • Sporulation at the source
  • Potential for spore survival during transport
  • Potential for spore deposition
  • Conditions for infection

In most cases, there is ongoing sporulation at a disease source and conditions will be suitable for infection if (or when) spores are deposited at a target area. Thus, risk assessments are frequently determined by the potential for survivable transport and deposition of airborne spores. The direction of the trajectory is also quite important, obviously, because only exposed cucurbit fields are going to be at risk.

Spore survival is a difficult and inexact problem. Exposure to several hours of direct sunlight kills airborne spores. Cloud cover shields airborne spores and allows them to survive. These are the guidelines we use:

If the forecast reads "fair", "mostly sunny", or "sunny", then sky conditions are unfavorable and we assume nearly all spores will die. If the forecast reads "partly cloudy" or "partly sunny", then sky conditions are mixed and we assume some spores will live while others will die. If the forecast reads "mostly cloudy", "cloudy", or "overcast", then sky conditions are favorable and we assume most airborne spores will survive.

Potential for spore deposition (rainout or washout) is based on the forecast chance of rain along (and near to) the trajectory. The greater the chance of rain, the greater the potential for deposition. The following is a list of the words or phrases associated with the different % chance of precipitation.

  • 20% - slight chance or isolated
  • 30% - widely scattered or scattered
  • 40% - scattered, sometimes good chance
  • 50% - good chance
  • 60%, 70% - rain likely or showers likely
  • 80% to 100% - occasional rain, periods of rain, occasional showers, etc.

This brings us back to the risk assessments. The Low category is reserved for those situations in which all or nearly all of the factors are unfavorable. Similarly, the High category is reserved for those events in which all or nearly all of the factors are favorable. Both the Low and High categories are small. The remainder of the possible scenarios are placed in a broad category called Moderate. Here, the factors are mixed. Phrased as a percentage chance of disease development, the categories are:

  • Low: 0-20% chance
  • Moderate: 20-80% chance
  • High: 80-100% chance

For those situations where conditions are mixed, it is very desirable to attempt to give a better description than "Moderate", as that category has such a wide range. Fortunately, many times this is possible. If conditions are mixed and weighted toward the favorable side, we call that Strongly Moderate. If conditions are mixed but weighted toward the unfavorable side, we call that Weakly Moderate. If there is an even mix of conditions, or if we are unable to determine if the risk is stronger or weaker, then the description remains simply Moderate. The percentage breakdown for the risk assessments is now as follows:

  • Low: 0-20% chance
  • Weakly Moderate: 20-40% chance
  • Moderate: 40-60% chance
  • Strongly Moderate: 60-80% chance
  • High: 80-100% chance

These breakdowns are the ones we use in the forecasts. Below is a list of the risk assessments, followed by a weather forecast that is typical for that risk assessment.

  • Low: Sunny to partly cloudy and dry through the forecast
  • Weakly Moderate: Partly cloudy with a slight (20%) chance of rain
  • Moderate: Partly sunny/partly cloudy with a chance of scattered showers
  • Strongly Moderate: Partly cloudy with afternoon/evening showers likely OR
  • Strongly Moderate: Mostly cloudy with a 30-40% chance of rain
  • High: Cloudy with rain likely


Low Risk does not mean Zero Chance.

High Risk is not necessarily a guarantee of infection.

Although both are important, sky conditions are generally given more weight than the potential for rainfall. For example, if the weather forecast reads "Sunny this afternoon. Increasing cloudiness overnight with a good chance of showers by morning", growers along the forecast pathway will be at Low Risk, despite the good chance of rain later. The reasoning is that solar exposure over the course of an afternoon should be more than enough to kill any airborne spores, minimizing the risk.

Forecasts extend out 36-48 hours. The chance of rain may vary during this time. One must also account for washout of spores over time when assessing the risk for growing areas farther along a forecast pathway. Thus, the risk to growers may (and frequently does) vary over the course of an individual transport event. For example, the weather forecast may read, "Mostly cloudy with a good chance of rain this afternoon and evening. Slight chance of showers late tonight and tomorrow morning". Growers along the afternoon and evening sections of the trajectory would be at High Risk; growers along the overnight and morning sections of the trajectory would probably be at Moderate Risk.

We try to make the forecasts as objective as possible. However, some subjectivity will always be present. Current observations, confidence in the weather forecasts, and Forecaster experience are all relevant factors. Given the same set of weather forecasts, it's possible that different Forecasters may produce slightly different risk predictions from the same information.

Authored by: Thomas Keever, Forecaster/Meteorologist, NAPDFC

Last change: February 25, 2009