Seedling disease complex of cotton
Losses due to seedling disease in the High Plains are generally negligible; however, the potential exists for stand reductions to occur. A number of different soilborne pathogens including Pythium spp., Rhizoctonia solani and Thielaviopsis basicola are involved in the seedling disease complex. High soil moisture provides conditions conducive for growth of these microorganisms and may predispose the developing seedlings to infection by slowing plant growth.
While the past few years have been characterized by the hot dry conditions, frequent applications of irrigation required to promote stand establishment may foster conditions that are conducive for seedling diseases. Ideally, planting should take place when soil temperature at the 4-inch soil depth is above 65° F and favorable conditions are predicted. It is recommended that the highest vigor seed lots be planted first in the event that planting into cooler soils is necessary (to cover a large number of acres or to take advantage of soil moisture).
All commercial seed comes treated with fungicides with constituents that have activity towards the aforementioned pathogens. Additional fungicide options, referred to, as over-treatments are available as needed.
The root-knot nematode (Meloidogyne incognita) is widely distributed throughout much of the southern High Plains and is capable of causing significant yield loss. Symptoms associated with root-knot damage consist of poor vigor, stunting, yellowing of leaves and wilting. A characteristic feature of root-knot nematodes is the formation of galls that occur on the roots.
In addition, infected plants may exhibit nutrient deficiency-like symptoms, as M. incognita females feed on cotton roots and disrupt the plant’s ability to acquire water and nutrients. The amount of damage observed is more severe when nematode populations are high. Furthermore, this damage may be enhanced by other stresses such as drought or herbicide injury.
Several cotton varieties with partial resistance to root-knot nematodes are now available. The varieties Fibermax 2011GT, Deltapine 174RF, Phytogen 367WRF, and Stoneville 5458B2F have been evaluated in the past and are known to have partial resistance and/or improved tolerance.
Field studies conducted in 2013 indicate that other varieties such as Deltapine 1454RN B2RF and Stoneville 4946BLB2 also possess root-knot resistance and yield similar to the aforementioned varieties. Furthermore, Phytogen 417WRF greatly reduces nematode reproduction and may be an option for fields that are severely infested. Additional studies are slated for 2014 to compare the performance of these and other cotton genotypes under varying nematode pressure.
A second nematode, Rotylenchulus reniformis or the reniform nematode, can also result in similar symptoms; however, the distribution of this pest is limited to only a few counties in the region. Currently, there are no commercially available varieties resistant to R. reniformis, but breeding efforts are underway to introduce sources of resistance from exotic cotton species. Laboratory analyses may be required to differentiate the two species. Sampling at the end of the growing season will provide the most reliable results, as nematode populations are highest at that time of the year.
Crop rotation with a non-host is a good way to reduce nematode densities, but both M. incognita and R. reniformis populations can build up quickly the next year cotton is planted. With the loss of Temik 15G, chemical management options for nematodes are limited. Performance of the seed treatment nematicides, such as Acceleron-N, the Aeris Seed Applied System and Avicta Complete Cotton are somewhat inconsistent and should not be used stand alone in high-risk fields.
Foliar applications of Vydate® are labeled for use in cotton; however, research is currently being conducted evaluating usage rates and application timings in order to maximize efficacy and increase profitability. The soil fumigant Telone II has been used to successfully manage nematodes in the High Plains; however, usage is limited. This is due to availability and cost of the product, as well as constraints that affect application (i.e. specialized equipment and adequate soil moisture at the time of application).
A number of products are being tested for efficacy against cotton nematodes, but data is limited at this time. Information from these and other studies will be made available as the season progresses.
Verticillium wilt, caused by the soilborne fungus, Verticillium dahliae, is the most economically important disease in the region. The fungus infects plants through the roots early in the season, grows through the vascular system, eventually clogging the xylem elements. As a result, plants exhibit severe wilt symptoms during bloom, when the water demand of plants increases.
In addition, fruiting positions may be lost and defoliation may occur prematurely. Increasing disease incidence can negatively impact yield and fiber quality. To further complicate issues, the fungus is capable of surviving in the soil for extremely long periods of time as microsclerotia. As with nematodes, increased densities of these survival structures are positively correlated with disease incidence and yield.
Dr. Terry Wheeler, Texas A&M AgriLife Research Plant Pathologist, is monitoring populations of the fungus with regard to cultural practices, such as irrigation amount and crop rotation. Other factors, such as soil temperature are also being used to try and predict Verticillium wilt outbreaks. The ultimate goal is to develop management strategies that integrate several tactics to limit damage due to this disease.
Currently, variety selection is the primary option for managing Verticillium wilt. Results from research trials conducted in 2013 evaluating most of the common varieties sold in the High Plains are available and should be used when booking seed to plant in fields with a history of the disease. A complete list of the varieties evaluated and the relative performance of those varieties is available here. These and other studies will continue to be conducted over the next several years.
For more information on these or other cotton diseases, contact Jason Woodward at email@example.com or 806-632-0762.