Effects and cost comparison of different interstock sprout inhibition and removal treatments in top grafted citrus trees

Abstract：【Objective】Top-working is commonly applied in orchard for variety replacement. However, nutrient competition between grafted buds and the interstock sprouts (sucker) inhibits of the growth of the grafted buds. And high temperature often causes sunburn injury on the exposed branches (interstocks) in summer. The objective of this study was to explore the effects of different treatments on interstock sprouting, sunburn incidence as well as canopy recovery. The costs of different interstock treatments were also recorded and compared.【Methods】‘Tarocco'blood orange (Citrus sinensis, interstock) grafted on‘Carrizo'citrange [Poncirus trifoliate (L.) Raf.×C. sinensis (L.) Osbeck] was chosen to be top grafted with‘Longhuihong'navel orange. Interstock sprouting inhibition treatments included covering interstocks with weed barrier fabric, and painting interstocks with lime sulfur paint (5 kg of lime, 0.5 kg sulfur, 0.5 kg salt, 0.1 kg vegetable oil, 20 kg hot water) , or with a new composite coating paste (4 kg JS-991 polymer modified cementitious waterproofing, 2 kg portland cement, 3 kg talcum powder) . Another group of disbudding treatments, including bud cutting, bud nipping above first leaf, bud removal, and bud removal + NAA, were applied to study their effects on interstock sprouting, as well as interstock sunburn prevention and tree canopy recovery. The costs of different treatments were also investigated and compared.【Results】Interstock sprouting time of all treatments was delayed. The control and all treated interstocks sprouted many suckers at first and then decreased gradually. Compared with the control, the interstocks treated with weed barrier fabric produced fewest suckers with a decrease of 83.7%, and interstocks treated with lime sulfur paint and composite coating paste had a sucker decrease of 42.1% and 24.9%, respectively. The length of the first shoots of the grafted buds was significantly increased after the three interstock sprouting inhibition treatments. The length of the shoots and the number of leaves with different treatment were significantly higher than the control. The diameter of the first shoots under weed barrier fabric treatment was significantly higher than the others. But no significant effect was found on their second and third shoots. However, weed barrier fabric treatment weakened the growth of the fourth shoots of the scion. The greening process of leaves and crown recovery were also affected. Interstocks covered with weed barrier fabric showed severe sunburn injury and had an average of 4.7 sunscalds per tree, while in the lime sulfur painting treatment, the composite coating paste treatment and control, the numbers of sunburn scalds were 1.4, 0.8, 2.6 per tree, respectively.Twenty five percent of control interstock had bark necrosis induced by sunburn scalds. Weed barrier fabric treatment costed the most, followed by composite coating paste treatment, lime sulfur paint treatment, and control. The highest cost of materials and labor was weed barrier fabric treatment, and the disbudding cost was lower than in the control. Among the disbud treatments, bud cutting treatment and bud removal + NAA painting treatment exhibited a strong effect in inhibiting interstock regermination of bud primordium (RBP) , and had a RBP rate of 13.0% and 23.4%, respectively. However, bud nipping above first leaf treatment had a RBP rate of 69.2%, and bud removal treatment 77.3%. There was no significant difference between different disbudding treatments in germination beyond bud primordium. After removal of the first sprouting on the interstocks, the follow-up total sprouting rate was 295.8% for bud removal treatment, 224.6% for bud nipping above the first leaf treatment, 122.0% for bud cut treatment, and 99.5% for bud removal + NAA painting treatment. As for the cost of interstock sprout removal, bud nipping above the first leaf treatment was most costly, followed by bud removal + NAA painting treatment, bud removal treatment, and bud cutting. Weed barrier fabric covering displayed the highest sunburn incidence in summer, and cost too much for large-scale citrus production. Lime sulfur had a poor effect in preventing sprouting from the interstock and the coating fell off easily. Composite coating exhibited a relatively better effect in all aspects. The bud cutting treatment costed less and effectively inhibited the regermination of interstocks. However, more wound increased the risk of infection of citrus canker. Bud removal+2 000 mg· L-1 NAA treatment did not completely inhibit sprouting. Therefore, it is worth to examine the effect of higher concentrations of NAA.【Conclusion】Interstock covered with weed barrier fabric was not suitable due to higher incidence of sunburn injury on trunks as well as high cost. Composite coating paste treatment had a better efficacy to inhibit interstock sprouting, prevent sunburn, and promote canopy recovery with lower cost. Bud cutting or bud removal + NAA painting treatments showed good effects on sprout removal and was low in cost.