Histopathology of Seed-Borne Infections - Applied Research Center ...

154 Histopathology of Seed-Borne Infections5.7 IMPLICATION OF INTERNAL INFECTIONThe importance of seed-borne inoculum is in its ability to survive and affect seedand seedlings at the time of germination. Agarwal and Sinclair (1997) have tabulatedthe information on the longevity of fungi in seed. It varies for different fungi andhost contaminations from 1 year to more than 13 years. However, there is noinformation on the longevity of a fungus in different hosts or different fungi in seedsof a host. Information on the nature of infection, superficial or deep, and its survivalperiod in seed will be useful for deciding on control strategies.Naturally infected seeds cause mostly deleterious effects, viz., seed rot, earlydamping off, seedling blight, spots, and necrosis on cotyledons, leaf spots, spots andstreaks on hypocotyl, and primary root rot (Maden et al., 1975; Singh and Singh,1982; Agarwal et al., 1986). Seeds with heavy internal infection of fungi mostly failto germinate with the exception of a few specialized biotrophs, e.g., loose smutsand endophytes. Ascochyta rabiei, with localized deep infection on the seed surface,affects seedling emergence only slightly. The seedling symptoms develop as spotsand rotting on the lower part of the stem and wilting and dryness of the leaflets(Maden et al., 1975). Heavy infection of Macrophomina phaseolina causes failureof seed germination and browning and rotting of seedlings in sesame, whereasmoderate and weak infections produce diseased seedlings (Singh and Singh, 1982).Albugo candida survives on plant debris in soil (Butler, 1918; Walker, 1969).The pathogen is shown to be seed-borne in Canada (Petrie, 1975; Verma and Petrie,1975; Verma et al. 1975) and also in India (Verma and Bhowmik, 1988; Sharma,Agarwal, and Singh, 1997). The seed-borne inoculum remains viable and causesseedling infection (Sharma, 1989). Verma and Bhowmik (1988) and Sharma,Agarwal, and Singh (1994) failed to recover oospores from plant debris buried inthe field after 6 months and have argued that the seeds with contaminated andinfected oospores provide an important source of perennation and primary infectionof white rust disease in India.The location of pathogens in seed affects seed viability as well as control ofseed-borne infection. Fungal pathogens survive for a longer period in seed evenunder normal storage conditions, and under optimal conditions this is considerablyprolonged. The dormant mycelium and fruiting bodies (sclerotia and reproductivepropagules) occurring in seed tissues are difficult to inactivate or control by fungicidaltreatments. The deeper the location of the inoculum, the more difficult it is tocontrol. Routine treatments and dosages prove ineffective. Treatment, if given,should be tested using growing-on tests. However, an adequate knowledge of thebehavior of internal inoculum during storage seems essential before steps to controlit are taken. It is also known that the inoculum in seed may lose viability muchbefore the seed viability is seriously affected.Grain quality is also adversely affected in moderately and heavily infected seeds,particularly due to the depletion of food materials. Infection by mycotoxin-producingfungi may cause health hazards.