The grapes of wrath from mildew and mold
“He is trampling out the vintage where the grapes of wrath are stored.”
— Julia Ward Howe, “Battle Hymn of the Republic,” 1861
John Steinbeck’s wife, Carol, suggested the title for her husband’s novel “The Grapes of Wrath” (1939), which helped earn him the Nobel Prize for Literature in 1962. The phrase “grapes of wrath” comes from the first verse of the “Battle Hymn of the Republic,” a proclamation that the Civil War was God’s vengeance upon the nation for the injustice of slavery. The hymn went on to become a popular anthem of American patriotism.
Unfortunately, I have my own “grapes of wrath” to deal with this summer in the form of an angry-looking grapevine with discolored leaves, black-stained canes, and mottled, splitting fruit (see pictures on my blog).
My grapevine has powdery mildew, a fungal disease that is common in our area to table and wine grapes and many other fruit and vegetable crops. The fungus (Erysiphe necator) overwinters in crevices on the canes, and in springtime releases spores that cause infection. These primary infections then produce another spore, conidia, which can continue infecting the grapevine throughout the growing season.
As if that weren’t bad enough, split fruit is particularly susceptible to another common fungal disease, Botrytis cinerea, also called gray mold. This fungus can infect grapevines at any time during the growing season, but it most often occurs close to harvest time when berries are full of sugar and infections are easily spread by birds and insects. Gray mold disease causes grapes to become mushy; white grapes turn brown and purple cultivars turn red. The grapes will shrivel and eventually drop off.
Grapevines can be infected with more than one disease at a time, not to mention a host of insect pests: stink bugs, mealy bugs, spotted drosophila, spider mites and thrips.
So where did my grapevine go wrong? Where did I go wrong for my grapevine? Some of the problem admittedly stems from lazy gardening practices, but excessive heat and high humidity with little rainfall, such as the weather we’ve had lately, can also make grapevines more vulnerable to disease. I can’t control the weather, but I can use Integrated Pest Management techniques to minimize the chance of grapevine diseases next season. My plan of action:
Remove and dispose of diseased leaves and grape clusters. They should not be used for compost or mulch. This winter, prune away and dispose of canes that have been damaged by insects, disease or weather. Also get rid of older canes that have already fruited and are no longer productive (grapes form on second-year wood). During the growing season, prune back new shoots as needed to create more space for air circulation and for sunlight to penetrate.
To help avoid powdery mildew, ensure that spray from other garden irrigation does not mist the grapevine, which creates the ideal warm, humid conditions in which powdery mildew thrives. Covering the grapevine when grapes are more mature and full of sugar will provide protection from birds and insects transmitting Botrytis.
If fungal diseases persist next year, alternating applications of sulfur and organic or chemically processed fungicide sprays may be used at appropriate times during the growth of the grapes. However, fungicides with different active ingredients should be rotated, because fungal diseases quickly develop resistance to them.
Steinbeck used “grapes of wrath” in his novel to portray the anger and hopelessness felt by thousands of families who were forced to leave the dust bowl regions of Oklahoma during the Great Depression, only to starve in California where orchards and vegetable fields were fertile and productive but farmers wasted the crops in order to drive up prices.
In fact, Steinbeck was so critical of the capitalist system that drove such unjust practices that he included every verse of the “Battle Hymn of the Republic” in the first edition of his book to ward off accusations that he was a Communist. Steinbeck, too, knew when a healthy dose of prevention was called for.
— Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at email@example.com.