Job s tears (Coix lacryma-jobi) as host of Fiji disease virus and Perkinsiella vitiensis N. S. Prasad *, B. Croft, S. Johnson and S. Work * Sugar Research Institute of Fiji, Drasa, Lautoka, Republic of Fiji Islands Email nalini@srif.org.fj ABSTRACT Fiji leaf gall disease is a viral disease caused by, Fiji disease virus (FDV). The virus is transmitted by an insect vector known as a planthopper (in Fiji Perkinsiella vitiensis Kirk.). The hosts of FDV in Fiji are sugarcane (Saccharum interspecific hybrid) and duruka (Saccharum edule). Duruka is a native plant food for Fijians. Fiji leaf gall is currently managed in commercial sugarcane fields by roguing and resistant varieties. Sugarcane varieties are screened for resistance using an insectary-based resistance screening method. Job s tears (Coix lacryma-jobi L.), also known as wild maize, has been reported as a potential host of FDV and its planthopper vector. Job s tears has been observed to develop galls similar to the characteristic symptom of Fiji leaf gall in the field. In this study, we tested Job s tears as a host of both FDV and P. vitiensis. The results have shown that Job s tears did not develop symptoms of Fiji leaf gall whereas sugarcane varieties including the variety, Mana, developed typical gall symptoms. Job s tears is also not a host of Perkinsiella vitiensis. The planthoppers did not lay eggs on Job s tears and no nymphs were observed. INTRODUCTION Fiji leaf gall (FLG) is widespread in Fiji in the commercial sugarcane crops (Saccharum interspecific hybrid), chewing canes in native gardens (Saccharum officinarum L.) and duruka (Saccharum edule Hassk.). Duruka is a species grown for its aborted flower which eaten as a vegetable. The most popular commercial sugarcane variety in Fiji, Mana, is moderately susceptible. Significant resources are devoted to managing the disease through roguing teams and screening new varieties for resistance. FLG was first recorded in Fiji on commercial cane growing in the Rewa River district and is wide spread in the Fijian islands. FLG has threatened the existence of the Fijian sugar industry since the late 1800s. A clean seed program and release of resistance varieties (Badila and Pompey) brought the original outbreak under control in the early 1900s. The introduction of two susceptible varieties, Ajax and Pindar in the 1940s and 1950s, created another destructive epidemic. By the early 1960s at least 10% of the stools in a field had FLG (Egan et al. 1989). Replacement of susceptible varieties with the resistant varieties, Ragnar, Homer and Spartan, combined with the implementation of an intensive roguing program, reduced infection to a low level. FLG is caused by the Fiji disease virus and is spread by infected planting material and by planthoppers of the genus, Perkinsiella. In Fiji, the vector is Perkinsiella vitiensis. The planthopper prefers well grown cane, high humidity and protection from drying winds. Sheltered valleys, rivers and creek beds are ideal
for the planthoppers. The main diagnostic symptom is the presence of galls on the undersurface of the leaf blade and stunted plant growth. Duruka is highly susceptible to FLG and is often grown along river and creek beds. It has been observed that most FLG infestations in commercial crops occur in areas where Saccharum edule is planted (Tamanikaiyaroi and S. Johnson personal comm.). Eradication of duruka could create a social problem in Fiji because of its popularity in the community. Job s tears (Coix lacryma-jobi, also known as wild maize) is a tall grain-bearing tropical plant of the family Poaceae or grass family native to East Asia. It is used for food and medicine. It is listed as a serious weed in Polynesia, a principle weed in Italy and Korea, a common weed in Australia and Fiji and many other countries. Job s tears has also been suggested as a possible host of FLG. This plant shows leaf gall symptoms similar to those caused by FLG. The main aim of this project was to determine whether Job s tears is an alternative host of FLG and whether Perkinsiella vitiensis can complete its life cycle on Job s tears. METHODS Job s tears as a host of FLG Job s tears seeds were planted into pots and at the same time single-eye setts of the sugarcane varieties Mana and Kaba were planted into pots. The Erianthus maximum variety Fiji 10, which was infected with FLG, was planted into pots to breeding planthoppers. Adult planthoppers (Perkinsiella vitiensis) were collected from a commercial sugarcane field and were carefully placed in nytrel mesh cages containing pots of FLG infected plants. The planthoppers were allowed to breed on the infected plants until second instar nymphs were observed. The second instar nymphs from the infected Fiji 10 plants were collected and placed on healthy uninfected Job s tears, Mana and Kaba plants, approximately 100 planthoppers were placed on each plant. The planthopper were allowed to feed on the plants for 21 days. There were 6 replicates of Job s tears, Mana and Kaba plants and 10 standard sugarcane varieties with differing resistance to FLG (Table 1.). After the planthoppers were removed from the plants, the plants were inspected for FLG symptoms each day for 100 consecutive days. The number of days for 50% of the plants to show FLG symptoms was calculated (RD50). Job s tears as a host of P. vitiensis Adult planthoppers were collected from a commercial sugarcane field and released on healthy uninfected plants of Job s tears, Mana and Kaba for breeding and completing their life cycle. There were 3 replicates of each variety. Fifteen days after the planthoppers were placed on the plants the number of egg punctures in the midribs of the plants were counted. Twenty five days after the planthoppers were placed on the plants the number of nymphs were counted and after 40 days the number of adult planthoppers were counted. RESULTS AND DISCUSSION
Job s tears as a host of FLG The relationship between the RD50 (number of days for 50% of plants to express symptoms) and the standard resistance rating of the sugarcane varieties is shown in Figure 1 ( R-squared 0.9134, y=-6.4777x + 107.33). These results show that transmission of the virus was successful in the sugarcane varieties and the standard varieties reacted as expected. Figure 1. Relationship between rating of the standard varieties and the number of days for 50% of plants to show symptoms (RD50). Standard varieties with no symptoms at 100 days were given a RD50 of 100. FLG developed in 50% of the highly susceptible sugarcane varieties (rated 9) after 43 to 46 days and 100 percent of the highly susceptible varieties were infected by the end of the inspection period (100 days) (Table 1). No plants of Job s tears developed any FLG symptoms during the inspection period. This suggests that Job s tears is not a host of Fiji leaf gall. The two sugarcane varieties Mana and Kaba showed FLG symptoms after 79 and 94 days respectively. From the results of this experiment Mana would be rated intermediate to resistant and Kaba resistant relative to the standard varieties. Mana has been the most widely grown variety in Fiji for the last 40 years. It has shown some FLG infection in locations highly favorable for the disease but has not suffered significant yield losses. The results of this experiment would agree with the field observations. Table 1: Showing the disease rating Standards Resistance Rating a RD50 b % infected plants Pindar 9 43 100
Ragnar 1 100 0 Homer 1 100 0 Fiji 10 9 46 100 POJ - 3016 8 67 100 H51-5017 5 81 100 POJ - 2364 5 67 100 H46-2731 4 85 67 H52-3157 5 74 100 H53-3303 6 67 100 Job's tears ND 0 Kaba 94 100 Mana 79 100 a Resistance rating on 1 to 9 scale where 1 is highly resistant and 9 is highly susceptible b RD50 is the number of day for 50% of plants to show symptoms, standard varieties with no symptoms at 100 days were given a RD50 of 100. c No disease detected Job s tears as a host of P. vitiensis The adults of Perkinsiella vitiensis laid eggs successfully on the commercial varieties, Mana and Kaba (Table 2). No egg punctures were observed on Job s tears. No nymphs of Perkinsiella vitiensis were observed on the Job s tears plants. At the end of 40 days there were no significant differences between the numbers of nymphs on the two commercial varieties. Table 2. Breeding and survival of Perkinsiella vitiensis on Job s tears and two sugarcane varieties. Host Egg punctures Nymphs 15 days 15 days 25 days 40 days Job s tears 0 b 0b 0a 0b Mana 126 a 0b 85a 99a Kaba 101 a 32a 69a 92a CONCLUSION This experiment has clearly shown that Job s tears is not a host Fiji disease virus. In this study, no Job s tears plants developed Fiji leaf gall symptoms but susceptible sugarcane varieties were heavily infected.
This is an important finding because it shows that Job s tears will not act as a reservoir for the virus around sugarcane fields in Fiji. Samples of Job s tears collected from fields in Fiji with symptoms superficially like Fiji leaf gall were assayed for the virus at the Sugar Research Australia laboratory with a RT-PCR assay but no Fiji disease virus was detected (Thompson personal comm.). Job s tears is also not a host of Perkinsiella vitiensis. The planthoppers did not lay eggs on Job s tears and no nymphs were observed. ACKNOWLEDGMENTS This research was supported by Sugar Research Institute of Fiji with the support of our Chief Executive Officer Mr. Jai Shree Gawander and Our Finances and Administration Manager Mr. Sanjay Namal Prakash. REFERENCE Croft BJ, Piggin CM, Wallis, ES, Hogarth DM, eds. Sugarcane Germplasm Conservation Exchange. Report of an International workshop held in Brisbane, Queensland, Australia, 28-30 June 1995 Canberra, Australia: ACIAR, 51-54. Egan, B. T., Ryan, C. C., and Francki, R. I. B. 1989. Fiji disease. In: Diseases of Sugarcane Major Diseases. Ed. By Ricaud, C.; Egan, B. T.; Gillaspie, A. G.; Hughes, C. G. Amsterdam, The Netherlands: Elsevier, 263-287 http://en.wikipedia.org/wiki/job's_tears http://www.essortment.com/all/growingjobtear_rrbj.htm