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PYTHIUM DISEASE CATEGORY Seed decay Pre-emergent damping-off Post-emergent damping-off Root and crown rot of young plants Root rot of mature plants Foliar blight Vegetable soft rot TARGETED PLANT TISSUE Seed planted in the ground is invaded and rotted before the seed germinates. Seed germinates; root and shoot are infected and killed before seedling shoot emerges above the ground. Seed germinates; root and shoot are infected but seedling shoot is able to emerge above ground before collapsing and dying. Young plants are healthy while germinating and emerging; after plant establishment, roots or crowns become infected. Roots of mature plants can become infected later in crop development. This results in “root pruning” and subsequent reduction in growth, vigor, and yield. Above ground leaves, stems, shoots become diseased when the pathogen is splashed up onto foliage. Fleshy vegetables such as cucurbit fruits, sweet potato roots, and potato tubers develop soft decays. Table 1. Categories of Pythium diseases of row crops Continued from Page 12 of the roots and shoots that just grew out of the seed. This early disease stage is often called damping-off. Damping-off is further divided into two phases. If the newly germinated seedling is infected so early and so severely that it dies before being able to break through the soil surface, this situation is called pre-emergent damping-off. However, post-emergent damping-off occurs if the diseased seedling is strong enough to emerge above the soil surface, only to succumb and collapse shortly afterwards. Collectively, seed decay, pre-emergent damping-off, and post-emergent damping-off can result in loss of plants very early in the production cycle, causing stand loss in the field. Healthy seedlings that escape death at the seed and newly germinated stages remain vulnerable to this pathogen; established seedlings can still be infected and become stunted and die due to diseased roots and crowns. Older, established plants have escaped the damping-off phase that kills seedlings but can be subject to infections that prune back the roots, leading to reduced plant vigor and yield. For example, Pythium can cause late infections in cauliflower and result in weakened roots and poorly yielding plants. This soilborne pathogen can even cause a foliar blight of leaves and shoots, though this type of disease is not very common. Bits of soil carrying Pythium can be splashed or moved up onto foliage and cause blights on crops such as spinach and bean. <strong>Final</strong>ly, the fleshy parts of some vegetable crops are subject to Pythium pathogens. If in contact with infested soil, cucurbit 14 Progressive Crop Consultant March / April 2021
Pythium pathogens form thick-walled oospores that enable the pathogen to survive in soil for prolonged periods. When sufficient soil water is present, Pythium forms swimming spores that are released and search for host roots. Pictured here is a cluster of zoospores just prior to release. fruits, sweet potato storage roots and potato tubers can develop a soft, watery rot that will result in a non-marketable commodity. Of the hundreds of Pythium species worldwide, relatively few species infect row crops. These plant pathogens can be conveniently placed into two categories. One group consists of Pythium species that have a relatively narrow host range and infect only a few crops, with those few crops tending to mostly be within a particular plant family. Examples are Pythium mastophorum, which primarily infects celery and parsley (Apiaceae family), and Pythium uncinulatum, which reportedly only causes significant disease on lettuce (Table 2, see page 16). The second group contains Pythium organisms that have very large host ranges. The two main species, P. aphanidermatum and P. ultimum, both infect scores of plants, including dozens of vegetable and row crops. Continued on Page 16 Maximize Your Grape Yields in 2021 with the Proven Power of Kimzall® Plant Growth Regulator Whether you’re dealing with Wine, Table, or Raisin Grapes, Kimzall will maximize both the yield and quality of your grape crop. Don’t expect Mother Nature to do all of the work, let Kimzall® take your grape production to the next level! Contact Ben Letizia for more information 559-284-1392 or benl@kimc1.com © 2021 Kim-C1, LLC. Kimzall is a registered trademark of Kim-C1, LLC March / April 2021 www.progressivecrop.com 15
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Page 41 and 42: References Biofertilizers Market Si
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