Fungi in Soybean Seeds: Detection, Importance and Control | [Hongos en semillas de la soja: Detección, importancia y control]
2005
Pereira Goulart, A.C.
Fungi in soybean seeds: detection, importance and control. The sole indication of purity, germination and vigor indexes in a seed lot is not sufficient to characterize its true class. One should also take into account the sum of the physical, genetic, physiologic, and sanitary traits to better characterize a seed lot. The sanitary condition is extremely important if one considers that many pathogens may be seed borne. If a pathogen is taken to the field with the seed, it can reduce the seed germination and vigor indexes and originate disease primary foci. As for the soybean diseases, most of them are incited by seed-borne pathogens. Among these, the most important are: Phomopsis spp. anamorph of Diaphorte spp. (cause of stem canker, stem and pod blight, and seed deterioration); Cercospora sojina (frog leaf spot); C. kikuchii (purple seed and leaf blight); Colletotrichum truncatum (antracnosis); Fusarium semitectum (seed deterioration); Aspergillus flavus (seed deterioration in storage); Sclerotina sclerotiorum (stem and pod white mold); and Rhizoctonia solani (seedling blight and dead patches). These pathogens are detected and identified by a specific test (Sanitary Test or Seed Pathology Test). The main purpose is to determine the sanitary class of a seed sample. This test is performed at a laboratory which must have the following equipments: growth cambers, binocular stereomicroscope, compound microscope, test boxes (gerbox and plastic or glass Petri dishes), substrate (filter paper), transfer chamber, sterilizing oven, autoclave, and other small tools (forceps, knives, scissors, and transfer loops). Several lab tests may be used to characterize the sanitary class of soybean sedds; The commonly used and internationally recommended is the filter paper method or bltter test which can detec all fungi in the tested seeds (internally and externally). Others tests (deep freezing method and agar incubation method) may be used. Soybean seed treatment with fungicides is one of the most effective ways to control seed- and soil-borne fungi, as well as Rhizoctonia solani damping-off. This practice represents only 0.6% of the total crop yield cost. The correct choice of the fungicides depends on the organisms to be controlled, due to differences in the performance of different fungicides in relation to different fungi. The combined use of systemic fungicides (which are absorbed during the germination and emergence process, controlling the fungi inside host plant tissues) with protectants or contacts ones (which are not absorbed by plants, but are effective against some soil-borne fungi and those that are on the seed surface) has been an effective strategy in the control of seed and soil-borne pathogens. Thus, best seedling emergence in the field has been observed due the use of fungicides mixture, in comparison with the use of only one fungicide.
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