1 Introduction

The scientific name of the plant species known by the common name “tasi” in the pampean region in Argentina and “moth plant” in New Zealand is a subject of controversy. Many authors refer to it as Araujia sericifera Brot. following Forster and Bruyns (1992) who considered A. hortorum to be a synonym of the former but in most floristic works in Argentina they are treated as two separate species (Villamil and Barton, 2009). The name A. sericifera Brot. has been used for moth plant in New Zealand (Winks and Fowler, 2000), but A. hortorum is now the preferred scientific name (Champion et al., 2010) and is the one we have adopted here. This species is native to South America. It is a perennial, fast-growing, broad-leaved herbaceous climber with twining stems that exhibits, as do the other members within the Apocynaceae, elaborate floral features, milky latex and seeds with an apical tuft of long silky hairs (Bucciarelli et al., 2008; Endress and Bruyns, 2000). In its native range this plant is appreciated for its medicinal properties; its edible fruits; and its ornamental value (Bayón and Arambarri, 1999). Because of the latter, it has been introduced into many countries where it has since become cause of concern owing to its high invasive potential and capacity to generate serious environmental problems (Csurhes and Edwards, 1998; Waipara et al., 2006). Vines can grow over other plants, even shrubs and trees (Fig. 1A), sometimes getting to completely cover and smother them or breaking branches because of their weight. They are also strong competitors for water and nutrients. Their caustic milky sap can cause skin or eye irritation (Elorza et al., 2004).

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Fig. 1

Araujia hortorum. A. Plant growing in its natural habitat. B, C. Disease symptoms on leaves (B) and a stem (C).

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It is an important weed in plantations of citrus in California (Spellman and Gunn, 1976) and of kiwi-fruit in New Zealand (Kiwifruit Vine Health, 2016), but it is mainly considered an environmental weed in some countries of Europe, Israel, South Africa, Australia, and New Zealand (Coombs and Peter, 2010; Csurhes and Edwards, 1998; Elorza et al., 2004). In New Zealand A. hortorum has been targeted for classical biological control (Waipara et al., 2006; Winks and Fowler, 2000). The rust Puccinia araujiae has been proposed as a biological control agent and permission to import and release it in this country has been granted (Anderson et al., 2016).

Fairly little is known about other fungal pathogens associated with the target species in its native range in Argentina. There are only a few old records of causal agents of spots on leaves, stems and fruits in the literature (Spegazzini, 1880, 1882; Viégas, 1961) and other more recent ones that have resulted from surveys carried out with the aim of identifying suitable biocontrol agents to be applied in New Zealand (Waipara et al., 2006). The purpose of this investigation was to study in detail one of the diseases encountered during such surveys selected because it was frequently encountered and because it was observed to be particularly damaging to its host at some collection sites.

2 Materials and methods

2.2

2.2 Isolation and identification of the pathogen

The pathogen was first found in association with large necrotic leaf spots showing concentric growth rings affecting plants growing near the town of Carlos Casares (−35.578°S, −61,126°W). This site is arrowed in Fig. 2. The fungus was isolated by transference of fruiting bodies (pycnidia) found on the spots to potato dextrose agar (PDA) with a sterile needle. Pycnidia were subjected to superficial disinfection prior to transference. To this end they were submerged in pure ethanol for one minute, rinsed in sterile distilled water, then submerged in sodium hypochlorite for two minutes and finally rinsed again in sterile distilled water (Yuan and Mohammed, 2002). In order to achieve the identification of the pathogen, pycnidia obtained from diseased plant material were cut by hand with a razor blade, and the resulting slices were mounted in water for microscopic observation and measurements. Some sections were stained with Phloxine B to facilitate the study of the conidiogenous cells.

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Fig. 2

Populations visited during surveys in Buenos Aires province. Sites where Ascochyta araujiae was/was not found are indicated by full/empty stars respectively. The arrow indicates site 10.

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3 Results

3.2

3.2 Identification and description of the pathogen

The pathogen was identified by the authors as Ascochyta araujiae Speg. based on the original description by Spegazzini (1882) as A. araujae.

This description is now amended and completed as follows: On PDA colonies show scarce, white to ochre aerial mycelium. On these, conidiomata with oozes of dark brown conidial masses, are formed in more or less regular concentric circles. On host tissue, conidiomata pycnidial (Fig. 3-A,B), mostly solitary, rarely confluent, semi-immersed, globose to subglobose, 80–180 µm diam. (mean = 19820 µm; n = 60), glabrous, ostiolate. Ostiole single, only slightly or not papillate, 16–25 µm diam. Pycnidial wall pseudoparenchymatous, composed of 3–6 layers of isodiametric cells disposed in textura angularis, 14–23 µm thick, outer 2–3 layers dark brown. Conidiophores absent. Conidiogenous cells enteroblastic, phialidic, hyaline, smooth, ampulliform, lageniform or doliiform, 5–9 × 3–7 µm (mean = 7 × 5.4 µm; n = 58) formed from the internal cells of the pycnidial wall (Fig. 3-C). Conidia subhyaline, with a faint green tinge, smooth, cylindrical to elliptical with rounded ends, 6.5–11 × 3–5 µm (mean = 8.6 × 3.5; n = 90), mainly 1-septate but sometimes 0 or 2 septate, not or only slightly constricted, egutullate (Fig. 3-D).

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Fig. 3

Ascochyta araujiae. A. Pycnidia developed on affected tissue. B. Microscopic detail of pycnidia and cirri. C. Pycnidial wall and fertile layer; the arrow indicates a conidiogenous cell. D. Conidia. E,F,G. Symptom development after week one (A), two (B) and three (C). Scale bars: A and B: 50 µm; C and D: 10 µm.

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4 Discussion

It was proven that the causal agent of the target-shaped leaf spot of A. hortorum is Ascochyta araujiae, a pathogen described by Spegazzini in 1882 as the causal agent of a leaf spot on A. albentis, a plant species that has since been synonymized with A. hortorum. A full description of both the pathogen and the disease is provided. The characteristics of the pathogen found in association with diseased material are in accordance with those in Spegazzini’s original description, but for the fact that pycnidia are twice as large. Such differences are to be expected since, according to Mel'nik (2000), the difference in size of pycnidia between specimens of a single Ascochyta species can be significant. The typical zonation of the cultures growing on artificial media in the laboratory is also a common feature in some Ascochyta species when exposed to alternating light and dark periods (Kaiser, 1973). The development of the disease was followed on artificially inoculated plants and this allowed for the association between the initial purple spots and the fully developed target-shaped spots to be made. This in turn led to the realization that the disease is more widespread than had been anticipated, and by far the most frequent one encountered during our field surveys on this host.

There are many reported studies on Ascochyta diseases in the literature, especially on those affecting crops in the Fabaceae (Hernandez-Bello et al., 2006; Mel'nik, 2000). The diseases they cause are known as ascochyta blights and represent serious limitations to legume production worldwide (Peever, 2007; Tivoli and Banniza, 2007). Ascochytoses can affect crops, forage and ornamentals. The development of resistant cultivars has shown to be the best long-term strategy for the control of ascochyta blights in crops. The use of fungicides for treatment of large areas is often not practical or even feasible, but for the control of ascochytoses on ornamental plants it is the most widely used method. Broad-spectrum fungicides with systemic activity, especially the ones in the benzimidazole group, have shown to be fairly effective to control ascochyta blights (Bretag et al., 2006).

The main adverse effect of diseases caused by Ascochyta spp. is a reduction in photosynthetic area due to necrosis (Mel'nik, 2000) and a decrease of photosynthetic efficiency of the green leaf area (Garry et al., 1998). The ability of many fungi to produce disease is strictly related to their ability to produce phytotoxins (Yoder, 1980). Although the pathogenesis mechanism has not been studied for A. araujiae, studies carried out on the related species A. rabiei, the causal agent of chickpea blight, indicate necrosis is produced prior to hyphal invasion of tissues, suggesting the participation of fungal toxins in pathogenesis (Kaur, 1995; Pande et al., 2005). The involvement of phytotoxins in pathogenesis has also been reported for A. pisi, the causal agent of leaf spot disease of pea (Abouzeid and El-Tarabily, 2003) and several other Ascochyta species (Andolfi et al., 2013; Masi et al., 2018). This kind of pathogenesis is considered characteristic of necrotrophs (Tivoli and Banniza, 2007). Necrotrophic plant pathogens kill host cells by means of toxic molecules and lytic enzymes and they subsequently decompose the plant tissue and consume it for their own growth (van Kan, 2006).

In accordance with knowledge on other Ascochyta species (Peever, 2007) it was found, through both artificial inoculation experiments and field observations, that foliage, petioles, stems and fruits of A. hortorum can all be affected by the disease. Pande et al. (2005) observed that in chickpea plants affected by A. rabiei, parts above girdled stems are killed and break off. Stem lesions did not girdle in our experiments, but petiole ones did and caused premature fall. The disease is widespread throughout the natural distribution of its host in the province of Buenos Aires, albeit causing significantly different levels of damage between locations. This widespread distribution might be explained by the disease being dispersed with the seed, as is known to occur for other Ascochyta diseases, for which infected seed can be the most important source of inoculum for long distance spread (Tivoli and Banniza, 2007). Notwithstanding, more studies are needed to determine the role of seed in the dispersion of the disease studied here.

Legume-associated Ascochyta species are host-specific. It has been reported that certain species can only cause disease on their respective hosts (Peever, 2007). In contrast, other Ascochyta species have shown a lower degree of specialization in host range tests. For example, Chiu and Walker (1949) found that A. cucumeris is able to produce disease to several host species within the Cucurbitaceae. This result is more in accordance with our findings since the isolate from A. hortorum was able to infect and produce disease on plants belonging to five other species within the Apocynaceae, two of them, Asclepias curassavica and Gomphocarpus physocarpus from a different subtribe than that of the preferred host. Further trials are planned to better assess the circumscription of the host range of A. araujiae.

Moth plant has become a cause of worry because of its invasive potential in New Zealand and other parts of the world where it has been introduced. As control using traditional methods is not always possible or successful, the feasibility of applying biological control is being investigated. In this regard, although the main interest as far as pathogens are concerned, is centered on the use of the rust Puccinia araujiae (Anderson et al., 2016) other complementary agents, such as A. araujiae, are being investigated. In this context, the results obtained from the specificity tests are relevant as it was shown that A. araujiae has the capacity of infecting and causing disease to G. physocarpus, known as “giant swan plant” and A. curassavica as “milkweed” or “bandera española”. Although both these species are exotic to New Zealand they are very much appreciated because they are the main hosts of larvae of the monarch butterfly Danaus plexippus (Elliott et al., 2009; Zalucki and Kitching, 1982) that has become an iconic species in New Zealand. In consequence, Ascochyta araujiae would most likely be disregarded as a potential biocontrol agent in this country.

5 Disclosure

The funding source had no involvement in the performance of this research. There is no conflict of interest.

6 Declarations of interest

None.

Author contributions

G.H.R.: Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing original draft.

F.E.A.: Conceptualization, Investigation, Methodology, Project administration, Supervision, Writing, review & editing.