Induction of Systemic Resistance against Charcoal-Rot of Cowpea Caused by Macrophomina phaseolina using Some Inducer Resistance Chemicals

Sixteen isolates of Macrophomina phaseolina were isolated from the roots of cowpea showing charcoal-rot collected from different locations in New Valley Governorate. All the obtained isolates were able to attack cowpea plants (cv. Balady) causing charcoal rot on the basal stem with various degrees of disease severity. Isolate No. 14 was the most aggressive one in this respect causing 85.8% charcoal rot. Potassium silicate (KS), propyl gallate (PG), hydroquinone (HQ) and salicylic acid (SA) at 1, 5 and 10 mM were used in this investigation as promising inducer resistance chemicals (IRCs) for controlling the disease in vitro and in vivo. Results illustrated that all tested IRCs had little effect on the growth of M. phaseolina at different concentrations in vitro. In addition, the inhibition of the growth was slightly increased by increasing the IRCs concentrations. Under greenhouse (New Valley Agric. Res. Station) and field conditions (New Valley Agric. Res. Station at El-Kharga and East Al Owainat Res. Station) all the tested IRCs significantly decreased charcoal rot compared to the check treatment (control). Both PG and HQ were the most efficient ones in decreasing the severity of the disease. On the contrary, both SA and KS recorded the lowest protection against charcoal rot severity. Also, all the tested IRCs significantly improved cowpea plant growth parameters, i.e. plant height, No. of branches/plant, fresh and dry weight of plants (kg/feddan) and yield components, i.e. pod length (cm), No. of seeds/pod, the weight of 100 seeds, total seed yield (Kg/ feddan) compared with control during summer season 2017. Cowpea seeds soaked in PG at 5 mM recorded the highest growth and yield components in both locations. While HQ recorded the lowest ones. Analysis of plant mineral compositions showed a significant increase in contents of nitrogen (N), potassium (K), phosphorus (P) and crude protein in cowpea plants raised from cowpea seeds treated with any of the tested IRCs compared with control plants during growing summer season 2017. The highest increase was obtained for these mineral contents when cowpea seeds were soaked in SA except for potassium in East Al Owainat. While, KS treatment recorded the lowest plant mineral contents of estimated minerals in both locations, except for potassium in East Al Owainat.
The activation of peroxidase (PO), polyphenoloxidase (PPO), phenylalanine ammonia-lyase (PAL), pathogenesis-related protein (chitinase and β-1,3 glucanase) and phenolic contents in inoculated plants with M. phaseolina treated with the IRCS were increased compared with untreated inoculated plants and untreated un-inoculated plants. PG recorded the highest levels of oxidative enzymes, pathogenesis-related (PR) protein and phenol contents during all tested periods of determination. In general, enzymes activities of PO, PPO β-1, 3 glucanase and chitinase begin to accumulate after two days of treatment and reached maximum levels at 8 days while PAL reached maximum level at 6 days then decreased progressively. On the other hand, total phenols increased in cowpea plants inoculated with M. phaseolina and treated tested IRCS. The highest accumulation of phenols was recorded 6 days after the application.