1. Introduction
Banana (and plantain,
Musa spp. L., Zingiberales: Musaceae) is among the top 20 food crops worldwide [
1]. The cooking banana types are among the most important crops cultivated for household food security and income generation by millions of smallholder farmers under subsistence-farming conditions in sub-Saharan Africa (SSA), where they are largely grown in mixed-crop fields and backyards of household compounds. Banana productivity in SSA remains low at 7.3 million metric tonnes/ha [
1], mainly because of the widespread negative impact of several endemic and exotic pests and diseases, including banana bunchy top disease (BBTD). BBTD is caused by the banana bunchy top virus (BBTV, genus Babuvirus) and is the most destructive virus disease of banana worldwide [
2,
3]. BBTD was first reported from Fiji in 1989 and is presently known to occur in 37 countries. In SSA, BBTD was first reported from the Democratic Republic of Congo. The virus is presently known to occur in 17 African countries (Available online:
https://www.bbtvalliance.org/index.php/bbtv (accessed on 2 March 2022)) where BBTV has since emerged as a serious threat to banana production [
3]. BBTV infection of banana plants results in a range of symptoms that generally culminate in a bunchy appearance at the top of severely stunted pseudostems [
4]. The virus-infected plants do not produce fruit when the infection starts before flowering, while late infections result in deformed and inedible fruits [
5]. Regardless of the time of infection, BBTV infection leads to 100% banana fruit yield loss from the infected plants [
6,
7].
BBTV is transmitted through vegetative propagation of infected banana propagules and by the banana aphid,
Pentalonia nigronervosa Coquerel (Hemiptera: Aphididae), which occurs on plants in the Musaceae [
8]. A closely related species,
Pentalonia caladii van der Goot, frequently found on plants in the Araceae and Zingiberaceae [
8,
9], has been shown to transmit BBTV under experimental conditions, albeit much less efficiently than
P. nigronervosa [
10]. Considering its poor BBTV-transmission efficiency and its generally restricted distribution to non-
Musa spp.,
P. caladii is unlikely to play a significant role in the natural transmission of BBTV in banana plantations. However,
P. caladii could be a significant vector in BBTV transmission to other hosts in which BBTV was recently detected, including
Heliconia sp. in Hawaii (USA) [
11] and
Alpinia galanga (L.) Willdenow and
Curcuma longa L. in Indonesia [
12].
The banana aphid transmits BBTV in a persistent, circulative, and non propagative manner [
13,
14], while there is no evidence for mechanical viral transmission [
13,
15]. The banana aphid can acquire the virus after at least 4 h of the acquisition access period (AAP) on infected tissue and requires a minimum inoculation access period (IAP) of 15 min to transmit the virus [
13]. The aphid retains the virus throughout its life. BBTV-transmission efficiency by
P. nigronervosa increases with increased AAP, IAP, virus titer in the source plant, and aphid abundance [
13].
Several cultural and chemical approaches were developed for BBTD management, including the use of virus-free planting material, quarantine measures, roguing of diseased plants, and use of pesticides to control the aphid vector [
16,
17,
18]. While these approaches were effective in large-scale monoculture banana plantations, they have not been widely adopted by smallholder farmers in SSA due to the low availability of virus-free planting materials, high costs of pesticide use for aphid control, and high labor requirement for rouging-based methods [
19,
20]. Host-plant resistance to the virus and/or the aphid vector, particularly in smallholder farming environments of SSA, offers the most economical and environmentally sound means for controlling virus diseases [
21,
22,
23,
24,
25,
26]. Previous studies focused on assessing
Musa genotypes’ resistance against BBTD [
5,
27,
28,
29], while resistance to the banana aphid has rarely been evaluated.
This study covers the evaluation of a set of
Musa genotypes representing all known
Musa ploidy levels and genomic groups (
Table 1) to identify resistance to BBTV and its banana aphid vector. Resistance to the banana aphid was evaluated in the absence of BBTV under a semi-controlled environment, while resistance to the aphid and BBTV was assessed over 36 months in the field under natural aphid colonization and BBTV infections. The results demonstrated the differential response of genotypes to both aphid performance and BBTD, and identified promising genotypes with high levels of tolerance to the virus vector and to the disease.
4. Discussion
Most banana cultivars originated from intraspecific or interspecific hybridization between wild diploid
M. acuminata (A-genome, 2
n = 22) and
M. balbisiana (B-genome, 2
n = 22) species of section
Eumusa, including diploid (AA, BB and AB), triploid (AAA, AAB and ABB), and tetraploid (AAAB, AABB, ABBB) variants [
35]. This study showed that in the screenhouse and the field trials, there was a wide variation in the performance of banana aphids on the
Musa genotypes with different A and B genome composition and ploidy levels. In the screenhouse, aphid densities on triploid and tetraploid genotypes were higher than on diploid genotypes. The rates of aphid population growth were faster and reached higher densities on two AAB triploids, Waema and Ebang, than any other genotype. Aphid densities under field conditions were relatively lower than in the screenhouse, but the trend of banana aphid performance on
Musa genotypes was similar under both screenhouse and field conditions. For instance, the AAB triploids Batard, Ebang, Essong, and Elat and the AAAB tetraploid hybrids CRBP 39, CRBP 969, and CRBP 535 were highly suitable to banana aphid population growth, resulting in the highest aphid densities both under field and screenhouse conditions. Aphid densities were lowest (about 10-fold lower population) on AA diploid genotypes Pisang Tongat, Figue Sucree, and Calcutta 4 compared with AAB and AAAB genotypes under field conditions. In general, aphids were more abundant on triploid and tetraploid genotypes combining both A and B genomes (AAB, AAAB) than on those combining only A (AA or AAA) or only B (BB and BBB) genomes. Only one genotype, Yawa 2, evaluated in this study corresponds to the ABBT group, which is a natural cross of section
Musa (
Eumusa) (AA and BB) × section
Australimusa (TT). This genotype under screenhouse evaluation supported high densities of banana aphids, but the genotype was not assessed under field conditions due to insufficient planting material.
Further studies are necessary to understand the underlying factors contributing to the differential aphid establishment rates in relation to host genomic composition. One aspect of investigation should focus on the thickness of epicuticular wax on leaf- petiole, and pseudostem. For instance, a thick epicuticular wax was reported to increase resistance to black Sigatoka of banana [
36]. Several studies have shown that successful aphid colonization and performance are affected by multiple factors, including (i) chemical content of the sap (e.g., nitrogen and carbon levels, and free-amino-acid composition in sap) [
37,
38]; (ii) defensive compounds that reduce aphid feeding and multiplication rate [
39]; (iii) plant physical properties, which serve as barriers to feeding and growth (e.g., leaf pubescence, smoothness or roughness of leaves, the presence of trichomes or the shape and color of the leaves) [
40]; (iv) leaf and plant color, which affect attractiveness and landing behavior [
41]; and (v) chemical cues affecting landing decision [
42,
43]. Further studies should consider comparisons of the physical and chemical properties of
Musa genotypes supporting low and high aphid population densities to understand the mechanisms contributing differential rate of establishment and population growth on different
Musa genotypes evaluated in this study.
As observed for most aphid species, apterous banana aphids were more abundant than alates in both the screenhouse and the field. Owing to their higher mobility, alates play a major role in the horizontal transmission of BBTV within and between the fields. Alate abundance is generally linked to increasing density of apterous forms. Consequently, a
Musa genotype that supports the establishment of high densities of banana aphids poses an increased risk for the horizontal spread of BBTV and heightens virus spread in the field. In the field trial of this study,
Musa genotypes with high BBTD incidence did not support relatively high numbers of alate aphids, and the genotypes with high alate populations were moderately affected by BBTV. Plant viruses are known to induce specific changes in the host plant, modifying the behavior of its vector, which may favor or impair virus transmission. A similar observation was made in a recent study that demonstrated differential emissions of volatile organic compounds (VOCs) by healthy and BBTV-infected banana plants of Williams (AAA) and a Pacific triploid (AAB) plantain [
42]. Relatively higher VOCs detected in the BBTV-infected plants were attributed to a stronger attraction to alate and apterous banana aphids than in uninfected plants [
42]. The diversity and concentration of VOCs were greater in the AAB plantain than in AAA Williams, which implies differential production of VOCs depending on the genomic composition.
The
Musa genotypes evaluated in the field showed large variations in BBTD incidence ranging from 0 to 100%. BBTD expression varied significantly with
Musa genotypes, with the highest AUDPC on the AA diploid Tapo, which conversely was among the genotypes with low aphid densities. This was followed by two AA diploids, Pisang Tongat and Figue Sucree; and two triploids, FHIA 25 (AAB) and Yangambi Km 5 (AAA). Generally, genotypes with only A genome (AA and AAA genomic groups) were more susceptible to BBTV infection, except for the wild diploid Calcutta 4 (AA), which was not infected after 36 months of exposure in the field, compared with genotypes with B genome. In regard to the genotypes with both A and B genomes, triploids can be found throughout the BBTV susceptibility spectrum; for example, the triploid FHIA 25 (AAB) was highly susceptible while PITA 21 (AAB) and Balonkawe (ABB) remained uninfected after 36 MAP in the field. In general, genotypes with more than one copy of the B genome (BB, BBB, ABB, and AABB) showed less susceptibility (0 to <20% incidence) to BBTV infection. For instance, low infection was recorded on the ABB triploids Daru, Pisang Awak, Fougamou, and BBB triploid, Lep Chang Kut. No infection was recorded on the wild diploid Balbisiana Los Banos (BB) and the triploid Balonkawe (ABB). This leads to the hypothesis that
Musa genotypes with two or more copies of the B genome possess better tolerance to BBTV infection. However, the exception was the synthetic hybrid FHIA 03 (AABB), which despite having two sets of BB chromosomes showed a relatively high BBTD incidence (62.5%). Another genotype, the synthetic hybrid PITA 21 (AAB), despite having only one copy of the B chromosome, showed a very high tolerance to BBTV infection.
Musa genomic studies can shed light on the potential role of the B genome in banana resistance to BBTD. The observations on BBTD occurrence on some of the genotypes in this study corroborate with previous studies [
28,
29].
Wild genotypes often harbour some traits linked to resistance to disease and/or pests, as observed in the present studies with Balbisiana Los Banos (BB) and Calcutta 4 (AA). The diploid Calcutta 4 is known to be resistant to black leaf streak disease (BLSD) caused by
Mycosphaerella fijiensis [
44]. Calcutta 4 (AA) has been used extensively in
Musa breeding as a source of black Sigatoka and BLSD resistance [
44] and for its partial resistance to banana weevil [
45]. Calcutta 4 has also been reported as resistant to some races of
Fusarium oxysporum f. sp.
cubense in subtropical Australia [
46], and
M. balbisiana accessions have shown resistance to Xanthomonas wilt in a greenhouse trial [
47]. The B genome is also known to confer some drought resistance in
Musa genotypes [
48]. The four genotypes—Calcutta 4 (AA), Balbisiana Los Banos (BB), PITA 21 (AAB), and Balonkawe (ABB)—that showed high tolerance to BBTV infection are of interest for breeding programs. PITA 21, a plantain hybrid developed by IITA and resistant to BLSD, is among hybrids grown by farmers in at least four countries in Africa, including Cameroon and Nigeria [
49], where BBTD is present. Balonkawe is a traditional landrace widely used in the Philippines. Both PITA 21 and Balonkawe can be used to broaden sources of resistance to BBTV. However, further research is necessary to assess the robustness of resistance by experimental inoculation of these plants with viruliferous aphids under controlled conditions.
The grouping based on reaction to banana aphid and BBTD identified a group of five genotypes that are highly susceptible to BBTD and less susceptible to the banana aphid, including Tapo, Pisang Tongat, Figure sucre, Yagambi Km5, and FHIA 25. The second group of 10 genotypes, Batard, Essong, Ebang, Elat, PITA 21, CRBP 39, CRBP 535, CRBP 838, CRBP 969, and Daru, were less susceptible to BBTD and highly susceptible to the banana aphid. These groupings indicate that BBTD incidence is a genotype trait and is not positively related to aphid abundance on a genotype [
29]. Although aphids were found on PITA 21 (AAB), Balonkawe (ABB), Calcutta 4 (AA), and Balbisiana Los Banos (BB), these genotypes were free of BBTV at 36 MAP in a BBTV endemic area. The lack of BBTV infection on these four genotypes could result from cases where aphids fed on them may not have been viruliferous, or the plants were difficult to infect by aphid inoculation. However, considering the establishment of the trial in a BBTV hotspot, with high levels of inoculum in the vicinity and the presence of spreader plants (BBTV symptomatic plants), and the same banana aphid population moving randomly in the field, there is a high probability that viruliferous aphids would have spread onto the plants of these four genotypes. However, due to evidently high tolerance, the plants remained uninfected. These genotypes may be resistant to virus inoculation. Hooks et al. [
5] stated that despite susceptibility to BBTV, some banana cultivars have some resistance to virus inoculation by the banana aphid. Further experimental inoculation with viruliferous aphids is important to understand the reason for the uninfected status of these genotypes, even after prolonged exposure to BBTV in an endemic location.