Since the last week of July we have had crazy weather here in the Ozarks.
Temperatures have often been below normal for this month, with observed high daytime temperatures in the low 70s to mid-80s, and above average moisture, at times occurring on an extended daily basis.
There has been flooding and resulting free standing water.
Although the summer has been extraordinarily pleasant for us, temperature wise, for warm season vegetable growers, delayed fruit harvest of tomatoes and weather-related disease pressure have presented a new set of problems.
It is not uncommon for vegetable gardeners to annually combat early blight of tomatoes and other diseases in the early growing season, but due to favorable environmental conditions this summer, late blight, an infamous devastating disease of tomatoes and plants in the Nightshade (Solanaceae) family, has occurred in Rolla, and most likely other parts of the state.
The notorious pathogen: Late blight (Phytophthora infestans):
Have you ever heard of the potato blight and associated Irish famine of mid-1800s Ireland and northern Europe?
Introduce the fungi-like organism Phytophthora infestans, commonly known as late blight of tomatoes and potatoes, the culprit for the Irish famine, and still a troublesome disease worldwide today.
Late blight is a pest of the Nightshade family, a plant family which has an extensive list of vegetable and annual plants commonly planted in gardens.
Tomatoes and potatoes are most susceptible, and weeds in the nightshade family can serve as a source of inoculum of not just late blight but other tomato and potato diseases including viruses.
Late blight can infect all parts of susceptible plants and can cause rot of harvested fruit.
When I came across this pathogen locally in Rolla, it was on a humid, cool morning during a week where light rain fell daily. The diseased tomato plants showed the classic signs (visual presence of the pathogen) of a white circle of spore producing growth around a grayish necrotic irregular spot.
Going back to the site a week later, the active diseased spots on the leaves had turned brown and necrotic and had shriveled.
More and more growth had been affected and was progressing through the plantings rapidly for "late blight may cause total destruction of all plants in a field within a week or two when weather is cool and wet [temperatures between 60-80 degree F.]." (G. Agrios. 424)
Advanced stages of late blight
University experts recommend using a multiple strategy "integrated" management plan which typically includes the following steps for control of late blight.
When planting potatoes, choose an available genetically resistant variety, and if in the future there are tomato varieties resistant to late blight, choose to purchase these.
Page 2 of 2 - Especially when weather predictions are favorable for disease growth, always conduct regular disease scouting, since prevention of the disease is critical for controlling its development.
Weeds can be a nuisance for more reasons than just growth competition with vegetable plants. They can unfortunately be alternate hosts of insects and diseases, and it is important to remove nearby weeds and discarded live plant material in the Nightshade family growing nearby.
But most important if late blight is in the area, use a responsible, regular application of a preventative fungicide. If using a protective fungicide, University Extension sources suggest using a product which contains the contact fungicide "chlorothalonil" to be most effective at preventing late blight.
Other products are listed for late blight control but are considered "fair" at prevention. These include products containing "copper" or the chemical formulations of "mancozeb."
Since late blight is so rapidly destructive and can spread miles away from your garden, don't delay, remove, burn or bury the plant material to prevent the spread. A good rule of thumb according to Cornell University Cooperative Extension fact sheet 726.20, if "5-10 percent of the foliage is infected," immediately burn or bury the plant (W. Fry 1998) to limit the spread.
Water splashing spreads the disease from plant to plant, and the use of drip irrigation or soaker hoses can lessen the amount of water splashing on plant foliage and limit the spread of the inoculum.
There is a lot of conflicting information available online to the home gardener on whether late blight can over season in the winter in the soil and on plant debris.
Traditionally growers were told that the strains commonly found could not, but unfortunately new strains have aided in the ability of the pathogen to survive the winter without a live host.
It is still considered uncommon, therefore keep in mind this possibility of over seasoning, but know that it is more likely you will be battling the strain which cannot over season the winter in the soil.
Vegetable gardening is a continual learning experience, especially when battling vegetable diseases. Weather patterns from year to year change drastically, and so do the diseases a home gardener will find in their gardens.
Knowing the right disease symptoms and signs to look for while incorporating an integrated pest management plan can prevent or limit the severity of plant disease outbreaks.