Weather Factors Considered in Each Phase of Disease Development


at the source and spore release

Dew period – 6 hr
Temperatures: 5 – 28C (41 – 82.4F); optimum 15 – 25C (59 – 77F)
Decrease in RH: is required for spore release

Examples: Foggy mornings, recent (past 12-24 hours) rainfall. The presence of extensive irrigation may provide sufficient atmospheric moisture to induce sporulation; this is a local condition that your Forecaster cannot predict. Lesser levels of sporulation are assumed to occur when temperatures are between 5 and 15C (41 – 59F). Persistent rain showers during spore release time can result in lower risk because of poor release and/or immediate atmospheric washout.


toward cucurbit-growing areas

TRANSPORT & SURVIVAL toward cucurbit-growing areas:

Shielding from exposure to ultra-violet (UV) radiation: 8 to 10 AM for 2 weeks
Relative humidity: Moderate to low decreases activity of sporangia

Examples: Clouds; cloudiness is the main factor considered because it provides UV blockage and lessens the drying effects of direct sunlight. Adjustments for UV blockage are fairly straightforward; accounting for desiccation is more difficult. Desiccation can occur even under cloudy skies. Both factors vary directly with the amount of sunshine received, so they are commonly considered together. They will usually be referred to as “effects of direct sunshine” or “effects of solar exposure.” When mostly sunny, both UV exposure and drying are assumed; if it’s mostly cloudy, then low to moderate UV exposure and little drying are assumed. The forecasts are constructed using these precepts as a guide.


along the forecast pathway

DEPOSITION along the forecast pathway:

Precipitation: Where? When? How much?

Examples: Rainfall. The forecasts are formulated based upon the probability that showers, rain, or thunderstorms may occur along the forecast track. The risk assessment varies according to the chance of rain (usually some %), the timing (before, during, or after passage of the spores?), the location (spores rained out before reaching production areas?), and the nature of the precipitation (thunderstorms? widespread light rain? etc…).


at cucurbit locations with newly deposited spores

INFECTION at cucurbit locations with newly deposited spores:

Temperatures ranging from 5 – 28C (41 – 82F); optimum 15 – 25C (59 – 77F)
Dew period: 2-6 hrs depending on inoculum concentration. Higher concentrations of spores require fewer hours of dew.

Examples: Fog, rain, daytime cloudiness, darkness. Fog and rain are the two main factors used to determine the free water requirement. The probability of fog and rain are commonly given in weather forecasts. Dew is also a provider of free moisture, but typically is not considered because: (1) it’s a “local conditions” phenomenon not usually mentioned in available weather forecasts, and (2) the scenarios in which it may be more important than fog or rain are rare. Daytime cloudiness and nighttime darkness result in low light conditions which favor infection in most cases. Sky conditions a few hours following suspected wet deposition are evaluated to determine if low light intensity will be present.