Late blight is an explosive disease. In humid, wet weather, and in the absence of fungicidal programs, this disease can quickly reach epidemic proportions and has the potential to literally wipe out a field within 24 hours. The disease has a notorious place in history, having wreaked havoc on Ireland’s potato crops in 1845 and 1846, causing widespread famine and the mass emigration of more than 1.5 million people to the United States.
In recent years, late blight has caused moderate to heavy economic losses in many San Joaquin Valley potato and tomato fields.
Late blight is a disease caused by a fungus commonly referred to as ‘water mold.’
Late blight is the common name for a devastating disease caused by a water mold fungus, Phytophthora infestans. As the name ‘water mold’ implies, this pathogen thrives in wet and humid conditions.
Such conditions are experienced in the San Joaquin Valley during the spring. It is at this time that spore formation and new infections are initiated for the current season. As weather warms and rains diminish, a temporary lull in pathogen activity occurs. With further development of the canopy, however, a microclimate is created which promotes further infection and growth. If the heavy canopy is combined with warm days and cool nights, we have the setting for an explosive outbreak of late blight.
There have been many springs in which our labs are inundated with samples that appear to have the white bloom of spores known as ‘sporangia,’ a typical symptom of late blight. However, wet springs also create ideal conditions for other phytophthora species to take hold. Three of the commonly isolated species in this region are Phytophthora capsici, P. parasitica, and P. infestans. In wet years, as the season progresses, we see more and more P. infestans infections.
Late blight infections can be explosive and widespread in wet years following extended droughts.
Frequently, when drought years are followed by sudden wetness, subsequent years produce heavy disease pressures. Such a scenario occurred during the transition from the 7-year drought of the late 1980s and early ‘90s into the unusually wet weather of 1995 and 1996. This set the stage for the unusually high disease pressures for the 1996 growing season.
In these circumstances, dormant or overwintering spores coming off a drought characteristically gain momentum and appear to grow with relative vigor. This characteristic is similar to that of diseases indigenous to cold climates experiencing winter snows. One oftentimes sees a surge of pathogen growth during the transition from the cold of winter to warm, spring weather.
Thus, by the time the 1996 season came, large inoculum levels had built up in the farming environment. The added rains experienced in the spring of 1996 accelerated spore pressures to an all-time high. Even those not involved in agriculture become aware of these conditions in the form of exacerbated allergies due to the high concentration of airborne fungal spores. For those with plantings near mountains, concentrations of spores were even higher due to eddying of wind currents, which concentrate particulates, including spores.
Even the worst cases of late blight are rarely caused by a specialized or resistant strain but are instead the product of favorable conditions.
During heavy fungal years, our lab is often asked to determine whether the late blight occurring in a field is the result of a fungicide-resistant strain or a highly aggressive strain. In nearly all cases, however, we find no signs of either characteristic. For instance, our investigations into the late blight of 1996 indicated that the primary culprits contributing to heightened problems with late blight were:
- Heavy inoculum loads or disease pressures.
- Excessive delay in the initiation of spray programs.
- Hot weather causing repeated abscission of flower sets, inducing more vegetative growth and an unusually lush canopy.
- Lack of complete spray coverage of tissues.
- Interim periods of weather with warm days and cool nights.
Additionally, once pockets of tissues were affected by P. infestans and arrested by fungicides targeted at late blight, these damaged and dead tissues became easily colonized by a host of secondary pathogens, such as Botrytis and Alternaria.
As you have likely noticed, many field and vegetable crops experience difficulties setting flowers when temperatures rise above 95 degrees Fahrenheit, especially when we have several hot days in a row. Under these conditions, crops such as beans, cucumbers, watermelons and tomatoes will typically drop their flowers. When this happens, much of the plant food and energy is diverted to vegetative growth. A thick canopy is difficult to penetrate when spraying materials. The situation is compounded by intense and continued heat waves, forcing growers to step up their irrigation programs.
With the added moisture, a thick canopy creating a humid microenvironment, interim switches in the weather to warm days and cool nights, and lack of complete spray coverage of fungicides, the setting is perfect for uncontrollable outbreaks of late blight.
Besides a sound sanitation program (involving removal of tomato and potato cull piles, volunteer plants, and wild hosts), a grower’s arsenal against late blight must rely upon a sound, protective fungicide program. Emphasis must be placed upon the idea of protection, rather than clean-up. In short, the best fungicide programs for late blight (and almost all diseases) are those which prevent the onset of disease. This is especially the case with the late blight pathogen, P. infestans, because it has a very fast rate of growth, and an initial infection can spread rapidly throughout the plant.
Furthermore, this pathogen can grow within as well as on the surface of tissues. Thus, topical sprays, while controlling surface growth of late blight, may leave internal infections unchecked. Systemic fungicides offer another dimension of protection on this latter issue. However, even the best of these superior materials will not be able to curb late blight, once set and growing in deep, subsurface tissues.
Using an air-blast sprayer to deliver fungicide is an effective means of controlling late blight.
One of the best systems for control of late blight relies on a staggered planting, which leaves periodic, open roads to accommodate a ground rig sprayer. The ground rig used is a specialized boom sprayer which delivers an air blast behind each nozzle (e.g. Willmar Air Trak, RoGator 854 Airtec, Spray-Air). The unit selected should have the capacity to deliver at least 20 to 30 gallons of liquid per acre. These are superior field and vegetable crop sprayers which ensure quality coverage and are analogous to the air displacement sprayers providing excellent coverage in orchards and vineyards.
This air-blast ground rig must be your foundation sprayer with alternative purpose (such as foliar nutrition), and/or quick emergency applications supplemented by fixed-wing aircraft or helicopters. We cannot expect miraculous feats from the arsenal of superior fungicides if we do not get the coverage necessary.
Another tool that has added dimension to spray coverage are the silicone-based surfactants (SBS). We have tested a few of these products against some of the more popular nonionic surfactants. We used polished glass underlain with a grid and at a set droplet volume, temperature and time period, and observed the area of effective spreading. In many cases the SBS outperformed even the finest non-silicone materials by 500% (i.e. 5x more coverage). It is important, however, that you consult your PCA for specifics on rates, forms, and combined materials, as tissue phytotoxicity may be a concern.
Finally, you must get your fungicide sprays on early, ahead of the disease, and with a goal of prevention rather than clean-up. Consult your PCA and chemical representatives on defining the effective period of protection, as influenced by varying sets of climatic conditions. Then, develop a periodic, protective spray program for the growing season. In all cases, institute sound, resistance management schemes.