the impact of pineapple mealybug wilt-associated virus-1 and reduced irrigation on pineapple yield

© Australasian Plant Pathology Society 2001 10.1071/AP00060 0815-3191/01/010031

Australasian Plant Pathology, 2001, 30, 31–36

The impact of Pineapple mealybug wilt-associated virus-1 and reduced irrigation on pineapple yield

D. M. Sether and J. S. HuA

Plant and Environmental Protection Sciences, University of Hawaii at Manoa, Honolulu, HI 96822, USA.ACorresponding author; email: [email protected]

Abstract. The impacts of Pineapple mealybug wilt-associated virus-1 (PMWaV-1) and reduced irrigation onpineapple fruit yield in plant and ratoon crops were evaluated in a field experiment in Hawaii. In the plant crop,PMWaV-1 infection and reduced irrigation had no significant effects on fruit weight. In the ratoon crop, plantsinfected with PMWaV-1 produced smaller fruit than disease-free plants and plants that received reduced irrigationalso produced smaller fruit than plants that received regular irrigation. Additive effects of PMWaV-1 infection andreduced irrigation were detected; plots of infected plants that received reduced irrigation produced the fewest fruit.Frequency distributions for fruit size in the ratoon cycle showed shifts to smaller sized fruit classes when PMWaV-1or reduced irrigation were present. Estimated yield from combined plant and ratoon crops for infected plants was6.7% less than disease-free plants when plants received regular irrigation. Reduced irrigation correlated with a 4.2%reduction in estimated yield of disease-free plants, and both infection and irrigation reduction together reduced yield13.4%. These results show that PMWaV-1 and reduced irrigation can have a negative and additive impact onpineapple fruit production in the first ratoon.

Additional keywords: closterovirus, mealybug wilt of pineapple.

IntroductionThe Pineapple mealybug wilt associated viruses (PMWaVs)

are closteroviruses associated with mealybug wilt of pineapple(MWP) (Hu and Sether 1999; Hu et al. 1997; Sether and Hu1999, 2000). PMWaV-1 is mealybug-transmitted (Sether et al.1998) and has been detected in pineapple (Ananas comosus (L.)Merr.) grown throughout the Hawaiian Islands (Gunasingheand German 1989; Hu and Sether 1999; Hu et al. 1997) and theworld (Borroto et al. 1998; Hu et al. 1996; Wakman et al. 1995).Although infection with one of the PMWaVs is required forMWP to develop (Hu and Sether 1999; Sether and Hu 1999,2000), plants can be infected with PMWaV-1 without exhibitingany apparent symptoms of viral infection (Hu et al. 1996, 1997).Infection rates in asymptomatic plants vary from 20 to 100%among clonal selections of pineapple in Hawaii. The lack ofoutward symptom expression caused by viral infection itselfmay have contributed to its worldwide distribution in pineapple.Asymptomatic infected plants that appear to be tolerant toclosterovirus infection have been, perhaps inadvertently,selected through the clonal propagation of crowns and otherpropagules from infected parent plants. However, PMWaVinfection may affect fruit yield or plant vigour, especially underan environmental stress such as drought.

Pineapple is a xerophytic, herbaceous perennial that isplanted year round and is the most economically importantagricultural commodity in Hawaii, USA. In 1998, 302 000tonnes of fruit with an estimated farm gate value of

US$92.8 million were harvested from 3400 ha (Martin andOsaki 1999). Pineapple can be grown in areas of low tomoderate rainfall (500–2000 mm/year) without irrigation(Collins 1960; Noffsinger 1961). However, the use of dripirrigation is now widespread in Hawaii. In Taiwan, a 30%yield increase was obtained when irrigation was appliedbiweekly during fruit development; similar increases wereobserved in Côte d'Ivoire, Guinea, and Hawaii(Bartholomew and Malézieux 1994). Pineapple is clonallypropagated, usually from the crowns formed on top of thefruit. The first production cycle, known as the plant crop,matures 15–20 months after planting of the crown. Afterthe plant crop harvest, one or more vegetative suckers areproduced by the parent plant and are the source of the firstratoon crop. The first ratoon crop requires 13–15 monthsuntil fruit harvest (Rohrbach 1991). Additional ratooncycles are possible only if the effects of MWP, nematodesand Phytophthora spp. are limited.

Diseases caused by other closteroviruses such as Beetyellows virus, Citrus tristeza virus, Little cherry virus, andSweet potato sunken vein virus reduce the yields of theirrespective plant hosts (Agranovsky 1996). Although there isinvolvement of at least one of the PMWaVs in MWP (Hu andSether 1999; Sether and Hu 1999, 2000), the impact ofPMWaV-1 on fruit yield in the absence of MWP, is also animportant concern to the pineapple industry because of thehigh PMWaV-1 infection rates in many of the Hawaiian

32 D.M. Sether and J.S. Hu

pineapple selections and the perennial nature of the crop. Inaddition, water resource conservation and management haveresulted in reduced irrigation rates in pineapple fields.This raises concerns about the effects of reduced irrigationon pineapple yields, especially if plants are alreadycompromised by PMWaV-1 infection. We report here theimpacts of PMWaV-1-infection and reduced irrigation onpineapple fruit yield in plants following the plant cropgenerated from crowns through the first ratoon harvest.

MethodsA strip-plot design with two irrigation treatments, regular or

reduced, and two subtreatments, PMWaV-1 presence or absence, wasused. Each treatment and subtreatment combination was replicatedfour times. Each of the 16 plots consisted of four rows 7.6 m long withplants grown on 27.5 cm centres. Plants in adjacent rows were offsetfrom one another. There were 106 plants per plot. Reduced irrigationtreatment was imposed by blocking the drip irrigation lines at 5months post-planting for a duration of 12 months during the plantcrop fruit production cycle and blocking at 20 months post-plantingfor a 6 month duration during the first ratoon cycle. Durations ofreduced irrigation in the ratoon crop were limited to preventpotentially severe losses to plants outside the experimental area thatwere on the same blocked irrigation lines. Rainfall and irrigationamounts were recorded daily for the experimental area. Irrigation (Lha–1 month–1) was applied as follows: (1996) September, 51 257;(1997) May, 38 646; September, 78 433; (1998) March, 41 074; April,27 694; May, 19 984; July, 12 139; August, 51 650; and November, 29068. Crowns used as planting material were a clonal selection from cv.Smooth Cayenne collected from a healthy, wilt-free plant crop field.Crowns were screened with PMWaV-1-specific monoclonal antibody35-6-5 in a tissue blot immunoassay (TBIA) (Hu et al. 1997) todetermine virus status of each crown, and sorted accordingly. MAb35-6-5 does not cross-react with PMWaV-2. Crowns were planted inMay 1996 and maintained with typical commercial plantationpractices. Ethephon was applied in May 1997 and July 1998 as incommercial plantations.

The plant and ratoon crops were harvested in November 1997 andApril 1999, respectively. Fruits were categorised into cannery sizeclasses and number and weights of fruits and crowns recorded for eachsize class. Fruit sizing categories were: fruit weight > 1800 g,1351–1800 g, 901–1350 g and < 450 g. Leaf samples from crowns wereassayed with PMWaV-1-specific TBIA to reaffirm virus status.

Data were analysed with SAS (SAS Institute, Cary, North Carolina,USA). Means and variance analyses were performed with the generallinear model procedure (proc glm) using orthogonal contrasts and testsof hypotheses. Where appropriate, weights were log transformed priorto analysis, and means separated with Waller/Duncan k-ratio t testswhich generate k ratios for Type I to Type II error weights. A k-ratio of100 (K = 100) is approximately equivalent to a significance level ofP < 0.05. Total fruit weight per plot, mean fruit weight per plot, andmean fruit weight in each size category were compared betweentreatments for plant and ratoon crops. Weights of fruit with crown, fruitonly and crowns only were compared between treatments for the ratooncycle. Estimated yields were calculated from individual fruit weightsproduced in each treatment and adjusted to tonnes of fruit per ha (t/ha).Size-category frequency distributions for fruit and crowns weregenerated from the percentage of fruit produced within each sizecategory for each treatment. Mann-Whitney tests at α = 0.05 were usedto compare the percentages of fruit within a size category from thereplicate plots between treatments and to compare the total number offruit harvested from each treatment (Devore and Peck 1986).

ResultsPlant crop cycle

High rainfall prior to September 1996 precluded the need forirrigation until 5 months after planting (Fig. 1). Irrigationapplied in September was followed by a month-long dry period(13.7 mm of rain), a month of average rainfall (61.2 mm of rain)then 2 months of unseasonably high rainfall, precluding theneed for additional irrigation until May 1997 (Fig. 1). Plots thatreceived reduced irrigation had two fewer irrigationapplications totaling 89 903 L/ha than plots that received regularirrigation (Fig. 1). No MWP symptoms were present in theexperimental plots for the duration of the experiment. Detectionassays specific for PMWaV-1 were performed at the plant andratoon crop harvests and did not indicate the presence of anynew PMWaV-1 infections in previously uninfected plants.Assays for PMWaV-2 had not been developed prior toconclusion of the study and thus no information is available forthe status of PMWaV-2 in the plants.

Mean plant crop fruit weights did not differ (P > 0.091)between treatments when fruit were separated intocommercial size categories or when combined. Frequencydistributions for plant crop fruit sizes did not differsignificantly between treatments (P ≥ 0.10). Estimated totalyields were similar for all treatments (Table 1).

Ratoon crop cycleIrrigated plots received three more irrigation applications,

totalling 88 752 L/ha, than plots that received reduced

Fig. 1. Total monthly rainfall (bars) during the plant crop and first ratooncrop cycles. The horizontal lines denote the time period in which the dripirrigation lines were blocked for the plots receiving reduced irrigationtreatment. Irrigation was applied through drip irrigation lines during themonths marked with arrows ( ). Ethephon applications are indicated bysquares (■).

Mealybug virus, irrigation and Pineapple yield 33

irrigation (Fig. 1). PMWaV-1 infection itself resulted in a15% reduction in estimated yield (P < 0.05) (Table 1).Reduced irrigation resulted in a 12.6% reduction inestimated yield across virus treatments (Table 1). PMWaV-1-infected plants receiving reduced irrigation yielded 25.5%(K = 100) less than PMWaV-1-free plants receiving regularirrigation (Table 1).

Treatment differences were detected when individualratoon fruit weights with and without crowns were compared

(Table 2). Infected plants produced fruit that weighed anaverage of 67 g less than uninfected plants (P < 0.05)(Table 2). Across virus treatments, the fruit produced byplants that received reduced irrigation weighed an average of59 g less (P < 0.01) than plants that received regularirrigation (Table 2). When irrigation and virus treatmentmeans were separated, the highest mean fruit weight wasproduced by uninfected plants that received regularirrigation and the lowest mean fruit weight was from infected

Table 1. Yields per hectare for plant crop, first ratoon and combined crops

Treatment Yield (tonnes/ha)A

Plant crop First ratoon Combined crops

Virus infection status

All PMWaV-free plots 74.60 a 65.95 b 140.55 b

All PMWaV-infected plots 71.07 a 55.96 a 127.03 a

Irrigation

All plots receiving regular irrigation 74.53 a 65.04 a 139.57 b

All plots receiving reduced irrigation 71.20 a 56.91 a 128.11 a

Irrigation × virus infection status

Regular irrigation/PMWaV-free 76.23 a 58.32 c 134.55 c

Regular irrigation/PMWaV-infected 72.78 a 54.67 bc 127.45 b

Reduced irrigation/PMWaV-free 73.82 a 55.98 bc 129.80 ab

Reduced irrigation/PMWaV-infected 68.39 a 53.00 a 121.39 a

AFruit weights from each treatment plot were pooled and estimated yields calculated based on plot size. Only yields within a treatment are directly comparable. Numbers followed by different letters within a treatment in each column are different at P < 0.05 (Virus infection status or Irrigation) or K = 100 (Irrigation × virus status).

Table 2. MeanA weights of ratoon fruits and crowns

Treatment Mean wt (g)

Fruit Crown


All PMWaV-free 1110 b ± 496 100 a ± 13

All PMWaV-infected 1043 a ± 474 104 a ± 14

Irrigation

All plots receiving regular irrigation 1107 b ± 514 103 a ± 13

All plots receiving reduced irrigation 1048a ± 454 101 a ± 14


Regular irrigation/PMWaV-free 1133 b ± 516 100 a B ± 12

Regular irrigation/PMWaV-infected 1077 ab ± 510 105 b B ± 14

Reduced irrigation/PMWaV-free 1086 ab ± 471 100 a B ± 13

Reduced irrigation/PMWaV-infected 1007 a ± 432 102 abB ± 13

AMeans were calculated using individual fruit weights (g) from each treatment and are followed by standard errors. Only means within a treatment are comparable. Means followed by different letters within a column are different at P < 0.04 for comparisons between virus infection status, P < 0.01 for comparisons between irrigation status, and K = 100 for comparisons between irrigation × virus infection status.BSignificant difference based on log transformed values.


plants that received reduced irrigation (K = 100) (Table 2).Infected plants also produced heavier crowns (K = 100) thandisease-free plants under regular irrigation (Table 2).

The total number of fruit harvested in the plant crop wassimilar between treatments, but in the ratoon crop, plots thatreceived reduced irrigation or that were composed ofinfected plants, produced fewer fruit (Table 3). The greatestnumber of ratoon fruit was harvested from plots ofuninfected plants that received regular irrigation and theleast number were harvested from plots of infected plantsthat received reduced irrigation (P < 0.05) (Table 3).

In the ratoon crop, there was a decrease in the percentageof fruit falling in the heaviest weight class when theadditional stress factors of PMWaV-1 or reduced irrigationwere present (Fig. 2A). Infected plants that received reducedirrigation produced the lowest percentage of fruit in theheaviest weight class (> 1800 g) (Fig. 2A). When infectionstatus subtreatments were combined, reduced irrigationresulted in a lower percentage of fruit in the heaviest sizecategory than did regular irrigation (Fig. 2A). Whenirrigation subtreatments were combined, infected plantsproduced a lower percentage of fruit in the heaviest sizecategory (Fig. 2A). Infected plants produced a higherpercentage of crowns in the highest weight class (> 101 g)than did disease-free plants, when plants received regularirrigation treatments and when irrigation treatments werepooled (Fig. 2B).

DiscussionIn the absence of MWP, infection with PMWaV-1

significantly reduced mean fruit weight in the first ratooncrop, but not the plant crop. Similar temporal patterns ofyield reduction have been observed in nematode impact

studies in pineapple (Sipes and Schmitt 1994). The loweryield of PMWaV-1-infected plants and plants that receivedreduced irrigation was partially manifested as a shift toproduction of a greater percentage of fruit in the smallerweight classes. Depending upon the desired market or use ofthe fruit, such a shift affects production economics. The yieldreductions observed were also due to the recovery of fewerfruit in the plots of PMWaV-1-infected plants and in plotsthat received reduced irrigation. It is not known if thereduction of fruit numbers was due to reduced numbers ofsuckers produced per plant, failure to set fruit, loss due toprecocious fruiting, or a combination of these factors.

Pineapple has the capacity to survive drought, but our studyconfirms that growth and yield reduction do arise whenirrigation is reduced (Black 1962; Ekern 1964). The appearanceof yield differences in plots that received reduced irrigationduring the ratoon crop may more accurately represent the effectsthat water reduction can have on the yields of PMWaV-1-infected plants. Unseasonably high rainfall may have maskedsome of the effects of this treatment during the plant crop cycle.In addition, the polyethylene mulch used in this study is usedthroughout Hawaii and other areas subject to drought tominimise water loss. It is likely that this plastic mulch limitedthe water deficit experienced by the test plants. Yield benefits ofirrigation have previously been demonstrated in many parts ofthe world, including Guinea, India, Taiwan, Côte d'Ivoire andHawaii (Bartholomew and Malézieux 1994). Studies on theeffects of cultivar on relative tolerance to drought have beenlimited. Based on differences in leaf reflectance of cvv. SmoothCayenne and Perolera before and after dehydration,Bartholomew and Malézieux (1994) suggested that cv SmoothCayenne may be more tolerant to drought. Similarly, cv. QueenVictoria is reported to be more drought tolerant than cv. SmoothCayenne (Sideris and Krauss 1934).

Under regular irrigation conditions, PMWaV-1 infectedplants in the ratoon crop produced larger crowns thanuninfected plants. It is unknown if a similar size differentialoccurred in the plant crop, because crown and fruit weightswere not separated. It is possible that the phloem-inhabitingclosterovirus may be affecting resource partitioning by theplant but further replications are needed under differentenvironmental conditions to establish a clear cause/effectrelationship between crown size and PMWaV-1 infectionstatus. The impacts that PMWaV-1 infection or reducedirrigation had on flavour, brix and other fruit characteristicswere not tested.

It is unknown whether further yield reductions could beexpected in additional ratoon cycles. The majority of fields inHawaii are not taken to the second or third ratoon stages asthey were several decades ago. This is partly due to thecompounding pressures of MWP, nematodes, andPhytophthora spp. that occur in these later cycles (Rohrbachand Apt 1986) and the lack of efficient, environmentally safe,control measures for these organisms. Reduction in thenumber of fruit produced by PMWaV-1-infected plants mayalso play a role.

Table 3. The number of fruit produced in the plant crop and ratoon crop cycle

Treatment Number of fruitA

Plant crop Ratoon crop


All PMWaV-free 845 a 815 a

All PMWaV-infected 850 a 736 a

Irrigation

All plots receiving regular irrigation 845 a 806 a

All plots receiving reduced irrigation 850 a 745 a


Regular irrigation/PMWaV-free 423 a 431 b

Regular irrigation/PMWaV-infected 422 a 375 ab

Reduced irrigation/PMWaV-free 427 a 384 ab

Reduced irrigation/PMWaV-infected 423 a 361 a

ATotal number of fruit produced in the respective crop from all combined replicates. Only numbers within treatments are directly comparable. Numbers within a column followed by different letters are different (P < 0.05).

Mealybug virus, irrigation and Pineapple yield 35

Since the conclusion of this study, we have found a secondclosterovirus in pineapple (PMWaV-2) (Melzer et al. 2001)for which we have produced a specific monoclonal antibody(Sether and Hu 2000). The PMWaV-2-infection status of theplants in this study, however, was not determined. Ourpreliminary data indicate that PMWaV-2 is not as widelydistributed as PMWaV-1 (D. Sether and J. Hu, unpublished).A badnavirus is also present in Hawaiian-grown pineapple(Hu et al. 1995). Studies in Australia previously indicatedthat badnavirus was present in all pineapple tested from theeast coast of Australia (Wakman et al. 1995; Thomson et al.1996). Based on this information it is likely that the materialused in the irrigation reduction/PMWaV-1 study describedhere was also infected with the badnavirus and some materialmay also have been infected with PMWaV-2 in either singleor mixed infections with PMWaV-1. Growing pineapple freeof the PMWaVs has two potential advantages. Firstly,

PMWaV-free pineapples are not susceptible to mealybug wilt(Hu and Sether 1999; Sether and Hu 1999, 2000). Secondly,PMWaV-1-free plants have the potential to produce higheryields than PMWaV-1-infected pineapple for combined plantand ratoon crops.

AcknowledgementsWe thank Dr H Fleisch, Ms L Kalua, Mr J Fleming and the

Maui Land and Pine Co. Agricultural Research Personnel atHonolua Plantation, Maui for their assistance and materials inkind. This research was funded, in part, by grants from theState of Hawaii Governor’s Agricultural CoordinatingCommittee contract No. 87-12, from the Hawaii Departmentof Agriculture contract No. 43754 and by the specificCooperative Grant agreement 58-5320-5-604 between theUSDA-ARS and the University of Hawaii. This is JournalSeries 4512 of the College of Tropical Agriculture and Human

Fig. 2. Frequency distribution showing the percentage of (A) total pineapple fruitproduced in five commercial size categories and (B) crowns produced in three sizecategories for each treatment during the ratoon crop. Distributions are shown for fruit fromplants that received regular and reduced irrigation when virus statuses were pooled, and forPMWaV-free and infected plants when irrigation treatments were pooled. Percentages ineach size category represent the pooled number of fruit from all replicates within atreatment. The percentage of fruit within a size category for each treatment replicate wasused for determining differences with Mann-Whitney tests. Bars labelled with differentletters within a category are significantly different (P < 0.05).


http://www.publish.csiro.au/journals/app

Resources. We thank Drs W. Borth, B. Sipes, K. Rohrbach,R. Paull, D. Bartholomew and M. Johnson and twoanonymous reviewers for helpful discussions and criticalreview of the manuscript and Dr J Silva for advice onexperimental design and statistics.

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Received 11 July 2000, accepted 14 November 2000

the impact of pineapple mealybug wilt-associated virus-1 and reduced irrigation on pineapple yield

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