Chlamydospores of P. chlamydosporia are considered as a useful source of inoculum for commercial production because they have high storage capacity and can be used at large volumes for inoculation in field conditions (Moosavi and Zare 2020). The success of a microbial nematicides in management of nematodes extensively depends on the strain, its infectivity, ease of mass production, and cheapness. One of the deterrents to the use of microbial nematicides is their inconsistent effect due to low quality, rapid decline of microbial propagules both during storage and after use in the soil, and instability when exposed to adverse environmental conditions (Moosavi and Askary 2015).
The composition of the culture medium can affect vegetative growth, colony appearance, sporulation, production of metabolites and pigment (Kerry et al. 1986; Liu and Chen. 2003). Carbon to nitrogen ratio of the substrate is an important factor that can increase spore production up to 30 times if appropriate. The C: N ratio is considered the best if it is around 10:1 (Mo et al. 2005). Also, the presence of substances such as starch and peptone is very effective in increasing the sporulation of this fungus (Sun et al. 2009). Flaxseed has a suitable ratio of carbon to nitrogen as well as containing high amounts of vegetable oils such as omega 3, starch and peptone (Goyal et al. 2014) and probably for this reason can be a more suitable substrate for sporulation of the fungus. However, the intrinsic ability to produce chlamydospores also vary among different fungal strains (Zare and Gams 2004).
The kind of substrate used for mass production is effective on the survival and development of the fungus in the later stages (Navarrete-Maya et al. 2017). So the chlamydospores produced on flaxseed and rice husk were tested separately on different carriers. Because chlamydospore survival was not statistically different, data were merged. The fact that the survival of chlamydospores produced on both substrates (rice husk and flaxseed) on different carriers (maize cobs powder, talc and kaolin) did not significantly differ indicate that the type of substrates used in this experiment had no or little effect on the amount of nutrient storage in chlamydospores, which are important for their survival.
The number of active chlamydospores decreased over time but the decline in the first month was very quick. This may happen due to the loss of a number of vulnerable fungal chlamydospores after being exposed to a new stressful environment.
Sawdust of maize cobs was not a proper substrate to induce chlamydospore production in the fungus (Luambano et al. 2019) but adding maize stover into soil integratedly with P. chlamydosporia reduce the root-knot nematode population in soil and on roots compared with when the fungus applied solely under glasshouse conditions. However the field experiment did not produce the same results (Luambano et al. 2015b). Soil amended with decomposed maize cobs powder resulted in better establishment of P. chlamydosporia in soil compare with when non-decomposed maize cobs powder added to soil (Luambano et al. 2015a). In our experiment the maize cobs powder-based primitive formulation had a significant better ability to retain the fungal spores for twelve months compared with talc- and kaolin-based primitive formulation. We used non-decomposed sterile maize cobs powder as an inert carrier for fungal chlamydospores and its decomposition started after added into soil. It has been reported that disintegration of organic matters with high C: N ration may lead to production of nematicidal and/or fungicidal compounds according to its original source (Moosavi 2022). It seems that decomposition of utilized maize cobs powder resulted in no or very little amount of the fungicidal compounds that do not interfere with P. chlamydosporia establishment.
According to current study, it can be concluded that IRAN 1212 C was better than the other two strains in terms of survival and sporulation on flaxseed and rice husk. In greenhouse conditions, maize cobs powder-based primitive formulation containing IRAN 1212 C had an acceptable performance in reducing nematode infection, suppressing M. javanica and improving plant growth. Due to the cheapness and abundant access to maize cobs powder as well as acceptable survival of chlamydospores, this material can be used as a suitable and inexpensive carrier for the formulation of this fungus. So it is recommended to use IRAN 1212 C and maize cobs powder carrier to produce a useful and efficient commercial product in controlling PPNs. Of course, additional field experiments on different hosts and nematodes can help confirm this result. As well, it is necessary to test proper additives to produce a formulation in which chlamydospore survive for longer time and tolerate higher temperature.