Page 55 - Free State Spring 2023_WEB

TOTAL PLANT MANAGEMENT A Stanton Gill

Basil Downy Mildew Field

Trial Evaluation- Year 3

T his article is for the benefit of greenhouse growers between major veins and eventually spreads across the

and garden centers.
leaf. As the disease progresses, the characteristic signs of
Commercially, basil is extensively grown for the pathogen appear as fuzzy grayish-purple sporangia
greenhouse and field production. Many greenhouses on the lower leaf surfaces. When humidity remains high,
do multiple planting dates to extend the market season the chlorotic lesions quickly turn brown and cause plant
for this valuable crop. High-volume retail basil sales at collapse.
garden centers, roadside farm stands, and farmer’s Downy mildew infects plants in a wide range of
markets make basil a very profitable crop for many temperatures, but infections are most severe from 59-68°
greenhouses and garden centers. People cannot seem to F. Basil downy mildew, specific to this crop, continues to
get enough basil for their gardens. be the biggest challenge to basil production in America
It looks and smells good, and you can obviously harvest because it can spread rapidly and result in complete
the foliage for culinary uses. After saying all these good yield loss. We usually see this sort of weather pattern
things about basil, we would like to concentrate on one in Maryland from late August through September. In
of the major problems with this crop. The dark cloud with 2022 we had these conditions in September. Under
growing basil is a disease - downy mildew- a relatively these conditions, downy mildew rapidly reproduces and
new problem. spreads, resulting in severe crop damage. Italian sweet
basil, which is the most popular for use in culinary dishes,
Basil’s downy mildew problems in the U.S. began in 2007 is highly susceptible to this disease. It also shows up on
in Florida during the 2007 growing season. Since 2008, lemon basil.
it has become widespread in many states and has been
reported in Canada and South America. The disease is It can be transmitted through seeds and transplants.
also widespread in Europe. New infections can start from wind-blown spores moving
northward from southern areas. As part of an IPM
Downy mildews tend to be specific to certain species approach, the nursery industry needs to think long-term,
of plants. Basil has its own downy mildew caused by involving breeding and selecting resistant cultivars.
the water mold Peronospora belbahrii. The pathogen
does not overwinter where freezing temperatures occur, Searching for Solutions
so the timing of infection proceeds from southern For the last three years, our IPM team has been evaluating
states northward each summer. Although the downy the new basil cultivar, Ocimum basilicum, for resistance
mildew pathogen has been detected in basil seed, to downy mildew. We published the results in GrowerTalks
seed transmission is probably a rare event. Air-borne magazine in 2021 and 2022. The best way to evaluate
dissemination of downy mildew sporangia, or transport of these new cultivars is to continue trials over multiple
infected plants, is the most likely way infection spreads to years where we experience different weather patterns
new sites. which influence the downy mildew infection. In 2022
Basil leaves that become infected with downy mildew will we expanded the number of cultivars in our trial and
develop a yellowish appearance, similar to a nutritional continued to test the ones involved in our 2020 and 2021
problem. Yellowing typically first appears in areas trials. (continued on next page)

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