Time of occurrence of hopperburn symptom on rice following root and leaf cutting and fertilizer application with brown planthopper, Nilaparvata lugens (stål) infestation

The manifestation of brown planthopper Nilaparvata lugens (Stål) hopperburn damage was investigated with damage due to rice leaffolder, Cnaphalocrocis medinalis (Guenée) simulated by leaf cutting and root pests by root cutting. Plants were grown under hydroponic culture, with a nutrient solution (PK) foliar applied to assess the effect on the onset of the damage symptoms. The effect of both root and leaf removal on the time (days) before hopperburn symptoms (DHS) depended on the rice variety. DHS was significantly shortened with TN1 variety (BPH susceptible), but not with root cutting. Removal of both leaves and roots from Xieyou 63 variety (BPH tolerant), did not affect DHS. However, multiple comparisons of means of combined treatments for leaf and root cutting showed that removal of two thirds of root and leaves significantly shortened DHS. Foliar P sprays with root cutting had no impact of DHS on TN1 compared to no spray, but with leaf cutting DHS of TN1 was shortened significantly. In contrast to TN1, foliar P sprays significantly prolonged DHS of Xieyou 63. For the two rice varieties, foliar K sprays did not affect DHS, nor did doubling P and K concentrations in hydroponics solutions with TN1 under root removal. There was a significant interaction between leaf removal and doubling P and K with leaf cutting, the DHS being shortest for the combined treatment of two thirds of leaf cut and double K. Doubling P and K concentrations did not affect DHS of Xieyou 63 with both root and leaf removal. The understanding of the relationship between DHS and root/leaf cutting, and application of P and K fertilizer for two rice varieties can provide valuable information for determining a control threshold of BPH when two pests coexist and for rational application of fertilizers. A high damage rate of rice leaves by C. medinalis can enhance the damage inflicted by BPH due to an interaction effect. Thus the control threshold of BPH should be decreased when C. medinalis outbreaks occur. In contrast, the threshold of BPH should be increased when there is a low damage rate caused by C. medinalis. The variety that delays DHS indicates that it has some tolerance to BPH infestation, thus control threshold should be increased or pesticide application be reduced.