A cool-season vegetable crop, spinach (Spinacia oleracea) seeds do not germinate when temperatures are above 86 degrees Fahrenheit, according to the American Horticultural Society's "A-Z Encyclopedia of Garden Plants." It appreciates germinating in a cool, moist soil. Unfortunately, many fungi also inhabit garden soils and can pose problems to the health of vegetables. Rhizoctonia is one fungus that causes a condition called "root rot" in spinach plants. Rhizoctonia, along with Fusarium, Pythium and Verticillium, can work in unison to harm growing plants in a field with poor drainage.
Rhizoctonia comprises many different species of fungi native to soils around the world. One species in particular, Rhizoctonia solani, is known to afflict many ornamental plants as well as common vegetable crops, according to a Western Farm Service publication. Regardless of species, these fungi persist in the soil indefinitely, affecting plants' leaves, stems or roots as weather conditions and plant susceptibility dictates. In spinach, Rhizoctonia is often simply referred to as "root rot."
In a garden or field sowed with spinach seed, one symptom of root rot is diminished or lack of seed germination. Closer examination of planting rows can reveal dead germinated seeds and wilted or rotted seedlings. In growing plants, there may be yellowing lower leaves or lesions on the stem at soil level. Looking at the roots you can see disintegrated or rotting, black roots, according to the University of California, Davis.
Rhizoctonia can manifest itself on spinach within one week's time, according to the Western Farm Service. Warming springtime temperatures accompanied by high humidity and/or abundant rainfall creates an environment that promotes the fungus' growth and potential to harming the spinach plants. Moreover, growing spinach in a greenhouse with overhead irrigation can just as easily promote fungal diseases.
Since Rhizoctonia fungus species persist in the soil, prevention is the key. Well-draining soil and appropriate watering diminishes the chance of fungus overgrowing to the detriment of the spinach crop. Both the University of California and the University of Delaware comment that no fungicides are effective against Rhizoctonia species (unlike Fusarium or Verticillium). Thus, the hope is to plant and grow spinach in soils not already infested with Rhizoctonia organisms. Where possible, use fresh topsoil or potting soil mixtures that you know to be free of the fungus.
Other recommendations to lessen problems with Rhizoctonia fungi is to rotate crops. Do not plant spinach for successive years in the same soil area in the production field or garden plot. Or, as the University of Delaware advises the home gardener, don't plant spinach in areas where parsley, cabbage (and other vegetables in the Brassicaceae family), potato or peas previously grew the past two or three seasons. These common vegetables also succumb to Rhizoctonia fungi when soil remains overly wet and promote the fungus' proliferation in the soil.