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 growth 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 (also a 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 and uneven plant stands. The overall effects of seedling root rots are often subtle, causing reduced seedling vigor.
The fungi that cause seedling diseases of vegetables are common pests to most all seeded plants. The most common are pythium species and Rhizoctonia solani. These are fungi that are common in most agricultural soils, including the soils of the San Joaquin Valley. Other fungi that infect vegetables seedlings are some specific species of rhizoctonia besides R. solani, certain fusarium species, and phylophthora species.
Wide host ranges
Many pythium species have very wide host ranges, including vegetables, and can be found in most all agricultural soils. Various pythium species can cause 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, in which case the plants in wet areas will be yellow and stunted.
Rhizoctoina solani is also very common in most soils and also has a very wide range of host plants that it can survive on. Different races or anastomiss groups (AG) of R. solani exists and more than one AG type can infect vegetables seedlings.
Symptoms of Rhizoctonia solani damping off of vegetables 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.
The classification of the genus fusarium is always complex and confusing, so it is suffice to say that there are various species of fusarium that have been shown to cause seedling diseases. However, it is important understand that when discussing fusarium that these fungi are very host specific as to which host a particular subspecies may infect. Also, fusarium is often times not the primary pathogen, but merely a strong secondary that may follow pythium or rhizoctonia, but in conjunction with these fungi help cause stand loss.
Because Fusarium spp. is often associated with these other fungi, their symptoms are all essentially the same. There may be more root discoloration, especially at the root tips, and colors such as purple and red may be noticed on the roots.
Making sure conditions at planting promote rapid seed germination and seedling growth can minimize the effects of seedling diseases. Plants can generally outgrow seedling disease problems when conditions favor rapid germination and growth. So proper growing conditions at planting is critical to avoid seedling disease problems.
Fields should be worked before planting so that water will be able to drain properly and any low spots removed. Compaction layers should be broken and low spots leveled before planting. Tilling will also encourage plant residue to break down, which may harbor the fungal pathogens.
Seed should be of high quality with the highest percent germination available seed used. Higher quality seed will generally produce more vigorous seedlings that will help outgrow 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, seed treatments are available for various vegetables. Some fungicides are specific as to the type of fungi they control, so one or more fungicides may be required. Fungicides can protect the seedlings from early disease problems and let the plant grow more vigorously in its early growth stages.