For the percentage of white tip symptoms, X. bovienii supernatant and P. lilacinum at 108 conidia ml−1 treatments were not significantly different than P. lilacinum at 106 conidia ml−1 treatment which had the lowest percentage of white tip symptoms (Fig. 1). However, in two out of three parameters (percent decrease in kernel numbers and percent decrease in panicle weight) measured, the X. bovienii supernatant and P. lilacinum at 108 conidia ml−1 treatments had the greatest effect on reducing A. besseyi infestation, whereas the other treatments gave more variable results (Figs. 2 and 3).
Figure 1 shows that the lowest percentage of plants with white tip symptoms was obtained from the P. lilacinum 106 treatment (19%), whereas the highest percentage symptoms was observed in the P. luminescens treatment (60%), followed by S. carpocapsae (57%) and control (51%), respectively. Interestingly, among P. lilacinum (all treatments), infected cadaver, S. feltiae, and X. bovienii treatments showed no significant differences, but all these treatments exhibited statistically less white tip symptoms than P. luminescens, S. carpocapsae, control, and H. bacteriophora (F = 7.33; df = 10, 54; P < 0.05) (Fig. 1).
The decrease of total panicle weight was the lowest at the X. bovienii treatment (8%) and the highest at the P. lilacinum 106 treatment (44%) (Fig. 2). Except at the P. lilacinum 108 and S. feltiae treatments, there was a significant difference between X. bovienii and the other treatments including control (F = 3.69; df = 10, 54; P < 0.05) (Fig. 2). The P. lilacinum 108 and S. feltiae treatments were not significantly different from each other.
For the percent decrease in kernel numbers, the X. bovienii supernatant treatment exhibited the strongest effect resulting in an average of 9% loss in kernels in each panicle, followed by 10% in the infected cadaver, 10% for S. feltiae IJs, and 22% for P. lilacinum 108 in treatments (Fig. 3). No statistically differences were observed among these treatments, but there was a significant difference between X. bovienii and all other treatments and control (F = 2.97; df = 10, 54; P < 0.05) (Fig. 3). However, the infected cadaver, S. feltiae IJs, and P. lilacinum 108 treatments were not significantly different than the other treatments, and the P. lilacinum 108 treatment was not significantly different than the other 10 treatments (Fig. 3).
Our findings indicated that several treatments had a significant negative effect on the rice white tip nematode. Evaluation of the treatments on three parameters showed that X. bovienii supernatant, S. feltiae IJs, and P. lilacinum at 108 conidia ml−1 consistently had the least effect on percentage decrease of kernel numbers and panicle weight loss as well as on white tip symptoms. The other treatments gave variable results although some of the treatments resulted in significant negative effects on the nematode for one or two of the parameters. For example, the infected cadaver treatment was equally effective as X. bovienii supernatant, S. feltiae IJs, and P. lilacinum at 108 conidia ml−1 on the nematode for percentage decrease of kernel numbers and white tip symptoms but not for percentage decrease on panicle weight.
Previously, different EPN species and their bacterial suspensions with different application methods have been tested on RKNs (M. incognita, M. arenaria, M. javanica, etc.), the ring nematodes, Criconemella spp., and the sting nematode, Belonolaimus longicaudatus. Most of these studies showed positive results, but some of them exhibited no beneficial effects (Grewal et al., 1999). Recently, Kepenekci et al. (2015) evaluated the suppressive effects of IJs and infected cadavers of four EPN species (S. glaseri, S. carpocapsae, S. feltiae, and H. bacteriophora) and two application methods (injection to soil and plant dipping) of X. bovienii or P. luminescens supernatants on the RKNs, M. incognita and M. arenaria, on tomatoes. Although the infected cadaver applications of EPNs and the aqueous suspensions of IJs suppressed the negative effects of RKNs, the most effective results were obtained from the dipping method of tomato roots in the X. bovienii supernatant. Also, our findings were similar in that the X. bovienii bacterial supernatant was one of the most consistent treatments, which resulted in the suppression of the negative effects of A. besseyi on rice plants. One common thread among the EPN species tested for control of plant parasitic nematodes has shown that S. feltiae was the most reliable in providing some level of control (Lewis and Grewal, 2005). Several mechanisms of nematicidal activity have been offered to explain how EPNs could have an effect on plant parasitic nematodes. Various predator/prey interactions may affect nematode populations indiscriminately (Ishibashi and Kondo, 1986). Both EPNs and plant parasitic nematodes are attracted to CO, produced by plant roots. EPNs may physically interfere with root invasion and feeding activities of plant parasitics (Bird and Bird, 1986). Grewal et al. (1999) showed that the S. feltiae associated with X. bovienii had a key role for the suppression of RKNs.
The highest rate (108 conidial concentration) of this fungus was as effective as that of X. bovienii supernatant for having minimal effects on percentage kernel loss and panicle weight. However, all rates of P. lilacinum were effective in reducing white tip symptoms. We hypothesize that there was no correlation between white tip symptoms and kernel loss or panicle weight.
Although there are no reports on the effect of P. lilacinum on A. besseyi, this fungus was applied to soil to control nematodes that attack plant roots (Esser and El-Gholl, 1993). In other studies involving P. lilacinum and plant parasitic nematodes, Khalil et al. (2012) evaluated the efficacy of azadirachtin 0.15%, azadirachtin 0.03%, oxamyl, Pseudomonas fluorescens, Bacillus subtilis, P. lilacinum, and abamectin against the RKN, M. incognita, on potted tomato plants. They showed that P. lilacinum (bio-nematon) was the most effective treatment for both egg and gall masses achieving 76.9 and 88.2% reduction, respectively. Siddiqui et al. (2000) demonstrated that P. lilacinum parasitized both eggs and females of M. javanica and that ethyl acetate extract of P. lilacinum and P. aeruginosa caused 100 and 64% mortality of M. javanica larvae, respectively, after 24 h.