Puccinia spegazzinii (Pucciniales: Pucciniaceae) from Peru for biological control of Mikania micrantha (Asteraceae: Eupatorieae) in India: evaluating susceptibility of host populations and confirming host specificity

Mikania micrantha Kunth (Asteraceae: Eupatorieae), a rapidly growing perennial vine from the Neotropics, is widely called mile-a-minute. It is an acknowledged invasive species in the Indian subcontinent, Southeast Asia and the Pacific region (Waterhouse 1994). In southwestern India, particularly in Kerala, it has been a persistent and expanding problem in natural forests, agro-forests and plantation crops. In the northeast, its importance has escalated in the last few decades due to large-scale degradation of natural forests, from where it spread to the commercially significant tea gardens (Murphy et al. 2000). In the biodiversity-rich Andaman and Nicobar islands, the unique native flora are under threat from the vegetation-smothering M. micrantha.

In Kerala, M. micrantha interferes with the cultivation and production of banana, coconut, coffee, cocoa, cassava, rubber, pineapple, ginger and teak. In the moist deciduous forests of the Western Ghats, the unhindered spread of M. micrantha makes harvesting of reed, bamboo and other non-wood forest products by tribal communities difficult. In homegarden farming systems in the Western Ghats region, M. micrantha was found to have an impact on production costs and income of all sizes of holdings, as it accounted for 10–20% of the total weeding costs (Murphy et al. 2000). On the other hand, in the northeastern state of Assam, the main concern is in tea plantations, especially in the light of herbicides being discouraged in this export-oriented crop. The weed has, to a considerable extent, prevented growers from swiftly changing to organic production.

Under a collaborative project between India and CABI in the UK during 1996–2000, the autoecious, microcyclic rust fungus, Puccinia spegazzinii de Toni (Pucciniales: Pucciniaceae), a common and damaging pathogen of M. micrantha in the Neotropics, was chosen as the most ideal candidate for introduction into India to manage the weed (Murphy et al. 2000; Sreerama Kumar et al. 2016).

Following a successful risk analysis under quarantine at CABI (UK), a pathotype of the fungus (IMI 393067) from Trinidad and Tobago was imported into India. In pre-release evaluation, the rust was further assessed under strict quarantine conditions to ascertain the susceptibility of M. micrantha populations from three affected regions of India and to confirm the safety of economically important plant species. Results of host-specificity screening of 90 plant species from 32 families confirmed that the Trinidadian pathotype of P. spegazzinii was highly host-specific and could not infect any of the test plant species, while it was highly pathogenic to most of the target weed populations from Assam, Kerala and the Andaman and Nicobar islands (Sreerama Kumar et al. 2016).

A supplementary dossier on the additional host-specificity tests provided the basis for obtaining the permit for release of P. spegazzinii from the Plant Protection Advisor to the Government of India in June 2005 (Sreerama Kumar et al. 2005).

The rust was first released in tea gardens in Assam in October 2005, but did not establish, most likely due to the presence of a biotype of the weed that was partially resistant to the Trinidadian pathotype used. In Kerala, the rust was initially released in agricultural systems, during August 2006, followed by releases at forest sites. Although on initial assessment those releases were considered to be successful because the rust had spread and persisted for a short while, there was no apparent establishment (Sankaran et al. 2008; Sreerama Kumar et al. 2016).

To supplement the Trinidadian pathotype, as well as to tackle some of the populations resistant to it, another pathotype of P. spegazzinii from Pilcopata in Peru was imported into India. In earlier investigations at CABI (UK), the Peruvian pathotype demonstrated broader pathogenicity towards both southwestern and northeastern populations of M. micrantha (Ellison et al. 2004).

The aims of the present work were to evaluate the susceptibility of M. micrantha populations from three different regions of India to the Peruvian pathotype of P. spegazzinii and to confirm its host specificity.

On M. micrantha, the initial symptoms of rust were apparent 5–7 days after inoculations, as chlorotic spots on leaves, petioles and along the stem. Within 12–15 days, the infection developed into dark orange-coloured telia with teliospores embedded in sori.

All the 12 populations of M. micrantha from the group of Andaman islands within the Union Territory of Andaman and Nicobar islands were found to be susceptible to the Peruvian pathotype, with the maximum mean pathogenicity score of 3.88 (Nayashahar) and the minimum of 3.50 (CARI-3) (Table 1). The rust pathotype produced normal life cycle on all the Andaman weed populations (Fig. 2).

The 15 populations of M. micrantha from Assam were found to be susceptible to the rust. Except Sepon, Silchar and Tezpur populations, the other 12 scored the maximum (i.e. PS 4) in pathogenicity (Table 1). The rust pathotype produced normal life cycle on the populations from Assam (Fig. 3). In contrast, there was remarkable variability in susceptibility of Assam M. micrantha plants towards the Trinidadian pathotype of the same rust (Sreerama Kumar et al. 2016). Even the M. micrantha populations from Karbi Anglong and Tinsukia that showed resistance to the Trinidadian pathotype were completely susceptible to the rust from Peru, with a median pathogenicity score of 4. In 11 populations, more than 50% of the leaf/petiole was covered with rust pustules. Fusion of pustules was seen only in Assam populations, specifically in those from Jorhat, Kokrajhar and Silapathar. Such fusion was not observed with the Trinidadian pathotype of P. spegazzinii (Sreerama Kumar et al. 2016). A comparative analysis at CABI (UK) indicated that hyperplasic canker production was high by the Peruvian pathotype but none by the pathotype from Trinidad (Ellison et al. 2008). Besides, the Peruvian pathotype showed a higher systemic spread than the Trinidadian pathotype. Substantial variation in aggressiveness of different P. spegazzinii isolates was also observed by Day et al. (2013a) in M. micrantha accessions from Fiji and Papua New Guinea (PNG).

All the three populations of M. micrantha from Kerala were found to be susceptible to the Peruvian pathotype with the mean pathogenicity scores of 3.90 (Chimmoni), 3.89 (Peechi) and 3.85 (Vazhachal) (Table 1). The pathogen produced normal life cycle on all the weed populations from Kerala (Fig. 4).

Twenty-five plant species, other than sunflower, belonging to ten tribes in the Asteraceae were found to be immune to the Peruvian pathotype of P. spegazzinii (Table 2). On the other hand, the pathogen formed normal pustules on all the positive controls.

In earlier studies with the Trinidadian pathotype, none of the 34 Asteraceae species (other than sunflower), including the present 25, reacted to the rust (Sreerama Kumar et al. 2016).

Fourteen out of the 31 sunflower cultivars/ accessions tested against P. spegazzinii (Peruvian pathotype) showed mild chlorotic flecks (PS 1) (Table 3) on a few top leaves that were directly below the heavy inoculum inside the dew chamber, around 6–8 days after inoculation. Out of those 14 plants, four had produced similar cholorotic flecks (Table 3) with the Trinidadian pathotype (Sreerama Kumar et al., 2016). On the other hand, six accessions that reacted against the Peruvian pathotype had not reacted to the Trinidadian pathotype, and three vice versa (Sreerama Kumar et al. 2016). Although EC-68414, a sunflower accession positive to the Trinidadian pathotype (Sreerama Kumar et al. 2016), was left out this time, four additional accessions were included and all produced chlorotic flecks with the Peruvian pathotype (Table 3). As expected, however, there was no sporulation in any of the cultivars/accessions. When microscopically examined, no mycelial growth was observed in the leaf tissue of the plants showing chlorotic flecks. The inoculated sunflower plants grew and flowered normally. This specific screening precluded the risk of the alien rust fungus posing any threat to sunflower cultivation in India. Ellison et al. (2008) also observed similar chlorotic flecks on sunflower in the primary host-range screening with the Trinidadian pathotype of P. spegazzinii in the UK. Observations till senescence indicated no mycelial growth in the leaves that exhibited chlorotic flecks.

Since ensuring host-specificity is paramount before releasing a biological agent into a new environment, P. spegazzinii (from Trinidad) was earlier rigorously tested and found to be safe to 55 economically important plant species in 31 families in India (Sreerama Kumar et al. 2016). After getting permission from the Plant Protection Advisor to the Government of India, the rust was first released in Assam in October 2005. The pathogen was released in Kerala initially in agricultural systems in August 2006, followed by forest sites. Kerala releases indicated potential, as the fungus was found to have spread to the native population of the weed at all sites. Field persistence, however, was observed only till December. Thereafter, microclimate at the sites was no longer appropriate for pathogen survival (Sankaran et al. 2008). Contrastingly, in Assam, the rust did not infect the M. micrantha plants in situ, possibly because of plant biotypic variation in susceptibility.

Experiences elsewhere indicate that simultaneous and sustained releases of P. spegazzinii at manifold locations and at appropriate time of the year could yield promising results. For example, between 2008 and 2013, an Ecuadorian pathotype of P. spegazzinii was released at multiple sites in PNG and Fiji. In PNG, from some sites, the rust had spread over 7 km within a year. In Fiji, the rust was established at 20 sites on two islands, i.e. Viti Levu and Vanua Levu (Day et al. 2013b). Continuous field studies in PNG indicated that P. spegazzinii could significantly reduce the growth and density of M. micrantha and offered great potential as a biocontrol agent.

In the present study, M. micrantha populations from across Assam, Kerala and Andaman and Nicobar islands were consistently found susceptible to the Peruvian pathotype of P. spegazzinii. In the host-specificity tests, this pathotype could not infect any of the 25 plant species, in ten tribes within the family Asteraceae indicating the safety of the pathogen. The development of chlorotic flecks on some of the 31 sunflower cultivars/accessions screened was apparently a hypersensitive reaction of no economic significance (Sreerama Kumar et al. 2016). It is deduced, therefore, that if released at an opportune time, the Peruvian pathotype of P. spegazzinii is expected to perform consistently in Assam, Kerala and Andaman and Nicobar islands as it is able to infect a wide range of populations of M. micrantha. Further, wherever required, it can be used alongside the Trinidadian pathotype.