Seedling disease can be caused by a variety of different fungi harming plants in the early growth stage. Seedling diseases are often a complex of two or more different fungi infecting a plant. But seedling disease is also a complex because it may appear at different stages of the young plant's growth.
Whatever fungi or what stage of the seedling is affected, nearly all crops propagated from seed face serious loss due to seedling diseases.
The effects of the seedling disease may appear as a seed rot (pre-emergence damping off), seedling decay before the seedling emerges (pre-emergence damping off), seedling decay after the seedling emerges (post-emergence damping off), or seedling root rot (root pruning). In pre-emergence damping off, the seed rots and never germinates, or the seed germinates but the seedling succumbs to the disease and dies before it can emerge. Post-emergence damping off is when the seedling emerges and then dies soon after. Whether the seedling disease is a pre- or post-emergence damping off, the results are thin or uneven plant stands. The overall effects of seedling root rots are often subtle, causing reduced seedling vigor.
The fungi that cause seedling diseases are common pests to most all seeded herbaceous plants. The most common are Pythium species and Rhizoctonia solani. These are fungi that are common in most agricultural soils. Other fungi that infect vegetable seedlings are Fusarium species, and Theilaviopsis basicola.
Wide host ranges
Many Pythium species have a very wide host ranges and can be found in most all agricultural soils. Various Pythium species can cause a seed rot and pre- and post-emergence damping off. Rotted seeds will be water soaked and mushy when squeezed. The roots of infected seedlings will be water soaked in appearance and gray in color. It may also cause a root rot in fields with poor drainage, with plants in wet areas being yellow and stunted.
Probably the most common species of Pythium infecting vegetables is P. ultimum, but there are many other species that have been associated with seedling diseases of vegetables. Pythium species generally survive as thick walled spores in the soil. Other types of spores can be produced, some which may swim to new areas on the plant root or to other nearby roots.
Rhizoctonia solani is also very common in most soils and also has a very wide range of plants that it can survive on. Symptoms of Rhizoctonia solani damping off are almost indistinguishable from plants infected by Pythium species. Symptoms include seed rot and damping off. The roots may be discolored, generally more so than with Pythium species.
Rhizoctonia solani survives in the soil as sclerotia on colonized organic matter from the previous crop. But it may also survive and reproduce on the roots of weed hosts. The survival of Rhizoctonia solani however depends on organic matter, either in plant debris or on living host such as weeds or a particular crop.
Making sure conditions at planting promote rapid seed germination and seedling growth can minimize seedling diseases. Plants can generally outgrow seedling disease problems when conditions favor rapid germination and growth of the plant. Unfortunately considerable amount of their early growth stage of vegetables is during the colder time of year. Proper growing conditions may be more critical for these two crops to avoid seedling disease problems.
Fields should be properly worked so that water will be able to drain and wet areas prevented. Compaction layers should be broken and low spots leveled before planting. Plowing the field before planting to turn under any previous crop residue will help lower the amount of Rhizoctonia spp. that may be present. Tilling will also encourage plant residue to breakdown.
To reduce the incidence of seedling disease, seed should be of high quality with the highest percent germination seed available used. Higher quality seed will generally produce more vigorous seedlings that will help out grow any seedling disease problems.
If possible, plant during a period that favors rapid germination and growth. Seeds that have to germinate during cool wet periods will have a tougher time than those planted in warmer conditions.
Avoid planting too deep. Seedlings are most susceptible to seedling disease from the time of planting to emergence. Seeds that are planted deep will be slower to emerge and more vulnerable to infection.
Finally, several fungicide seed treatments are available. Some fungicides are specific as to the type of fungi they control, so one or more fungicide may be required. Fungicides can protect the seedlings from early disease problems and let the plant grow more vigorously in its early growth stages.