Pest/Pathogen of the Month
Pest/Pathogen of the Month: February
Scientific name: Elsinoe necatrix
Common name: Eucalyptus scab and shoot malformation
Elsinoe was introduced by Raciborski (1900) to accommodate a fungus that caused scab-like lesions on plant tissue. The genus name Elsinoe, interestingly, was taken from the name of the heroine in the play Iridion by the Polish poet and dramatist Zygmunt Krasiński.
The serious disease, recently named as Eucalyptus scab and shoot malformation, was first observed in North Sumatra, Indonesia in 2014. However, the causal agent was discovered only recently as a novel species of Elsinoe. The name of the pathogen is derived from the Latin word “necatrix” (killer, murderess) to refer to the destructive impact that the disease has on its Eucalyptus hosts.
The disease is characterized by black necrotic spots that initially appear on young leaves and petioles, which become scab‐like as the lesions age. Infected trees respond to infection by producing shoots with small leaves that commonly appear feathered. Severely affected clones usually die after a number of successive infection cycles, generally over a period of two to three years.
The origin of Elsinoe necatrix is unknown, but evidence suggests that this is likely from an area where its Eucalyptus species are native. The pathogen would then have been accidentally introduced into North Sumatra where it has encountered susceptible host trees and an environment conducive to infection. In this regard, it should be considered as a high-risk pathogen and efforts should be made to prevent its spread to new environments.
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Pest/Pathogen of the Month: January
Scientific name: Pewenomyces kutranfy
Pewenomyces kutranfy is a recently-discovered fungal pathogen with the ability to girdle branches and trunks of the symbolic Araucaria araucana in Chile. This tree is of sacred importance to the indigenous Mapuche people in Chile, and from whose language, Mapudungun, the pathogen’s name was derived: Pewen being the name for A. araucaria, and kutran, meaning disease.
Pewenomyces is a monotypic genus inside the Coryneliaceae. This family contains other emergent canker pathogens including Caliciopsis pinea and C. Moriondi on Pinus species in North America and Europe respectively, and Hypsotheca pleomorpha on Eucalyptus spp. in Australia. Although Pewenomyces resembles these species morphologically, it is distinctly adapted to cold temperatures and, so far, has only been recorded from A. araucana. Research is still needed to disclose the rest of the story regarding P. kutranfy, although, the current hypothesis points to a native pathogen causing uncharacteristic disease epidemics as a result of climate change.
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Pest/Pathogen of the Month: December
Scientific name: Approximately 1300 species distributed among five plant families e.g. Arceuthobium spp., Viscum spp., Phoradendron spp.
Common name: Mistletoe
In Nordic mythology the goddess of love and fertility, Frigg, had a premonition that her son Baldur would be killed. She went forth to obtain an oath from all object in the nine realms that they would avoid harming her son, but she overlooked mistletoe due to its small and innocent demeanour. Even though Baldur was invincible, a mistletoe dart created by the god of mischief, Loki, killed him where he stood. Today, mistletoe is considered an obligate aerial parasite of several cornerstone tree species in forestry. Berries of mistletoes are eaten by herbivores that spread the seeds through defecation, especially birds. Seeds may also be disseminated by wind or hydrostatic pressure. The seed of mistletoe is coated by a sticky substance called viscin that cements the seed and ensures the seed is not disturbed. In spring, the seed will germinate and produce a haustorium that penetrates the vascular tissues of the host plant to syphon water and photosynthetic metabolites. Leaves of the mistletoe are dark green, oval and rigid and flowers in summer may be white, yellow or red. White berries produced by this plant contain toxic compounds that are poisonous to humans and some animals. Mistletoe has been reported as a serious parasite of conifers, especially in Northern America and Europe. Infection leads to weakened trees which reduces growth and timber quality, allows for attack by insects and bole swelling at the base provides a home for decaying fungi. Infection at the crown may also impact the soil ecosystem and the optimal functioning of the ectomycorrhizal community of the tree. In the end, the tree may succumb to the combined damage of the different factors and this may lead to extensive yield losses. Mistletoes are distributed across several different plant families, however, in the Southern Hemisphere most of the species belong to the Loranthaceae and Viscaceae. Despite their fragile exterior, the oath of the mistletoe should never be treated as superfluous.
Pest/Pathogen of the Month: November
Scientific name: Berkeleyomyces basciola and B. rouxiae
Common names: Black root rot
Black root rot is a fungal disease caused by two species from the genus Berkeleyomyces. These species are hemibiotrophic pathogens that penetrate living root tissue before killing them to obtain nutrients. Coalescing necrotic lesions lead to black discolouration of the roots, the character to which the disease attributes its name. Thus far, black root rot has been reported on more than 170 agricultural and ornamental plant species.
For nearly 100 years, the disease was thought to be caused by a single fungal species, Thielaviopsis basicola. However, in 2018 researchers at FABI showed that black root rot is caused by two cryptic sister species in a distinct generic lineage of the Ceratocystidaceae. Unfortunately, for most of the reported cases, the species responsible for the disease is unclear as identification relied on morphology rather than DNA evidence. With so little known regarding the true host range and distribution of these two species, concerns arise regarding their quarantine status and regulations surrounding their movement.
Pest/Pathogen of the Month: October
Scientific name: Cercospora zeina Crous & U.Braun
Common names: Gray leaf spot
In Africa, Cercospora zeina is the predominant causal pathogen of gray leaf spot disease of maize. This fungus is a threat to food security, since it can cause severe yield losses on both small-holder and large-scale farms. The foliar symptoms caused by this fungus are tan to gray rectangular lesions that are restricted within veins of maize leaves. After consuming its “maize meal” inside the leaf, the fungus bursts out of the leaf stomata with a fresh set of spores to infect the next leaf. Many things still puzzle scientists about this fungus, such as the questions “Where did it come from?” and “How has it moved around Africa?” Gray leaf spot disease was first reported in the 1980’s in KwaZulu-Natal, South Africa. Reports of the disease have since emerged from other maize producing countries in sub-Saharan Africa. Could this be due to international and local trade of maize material that may contain infected leaf sheaths and ear husks, or has it been hiding in a wild grass species? Molecular analysis has shown C. zeina to have high genetic diversity in Africa and that it undergoes cryptic sex. No one has yet observed its sexual stage, but we suspect that this contributes to its high diversity and aggressiveness. Clearly, there is need to design and employ integrated pathogen management strategies to limit its reproduction and dispersal to ensure optimal maize production and food security in Africa and globally.
Pest/Pathogen of the Month: September
Scientific name: Spongospora subterranea f. sp.subterranea (Sss)
Common names: Powdery scab of potato
Powdery scab, caused by the obligate plant pathogen Spongospora subterranea f. sp. subterranea (Sss), is an unsightly blemish disease on potatoes and is a major problem in the potato industry worldwide. Powdery scab is identified by purple-brown pimple-like lesions that rupture the tuber periderm, creating powdery filled lesions. The powdery mass consists of masses of sporosori (collections of resting spores). These resting spores are highly resistant to unfavourable environmental conditions, allowing the pathogen to survive in the soil for over 50 years. The presence of these lesions reduces quality and marketability of seed tubers or tubers intended for consumption, causing major yield losses in potato production. This plant pathogen is also responsible for causing two other diseases, namely root infection and root galling, which also lead to yield reductions. Powdery scab disease is most severe in fields when the soil temperature is cool (9-17 °C) and has a high water content. Although infection occurs under cool and wet conditions, diseases have been recorded in hot and dry climates too, especially where irrigation is applied. The host range of Sss is broad as it infects plant species belonging to at least 26 families. Many weeds and commercial crop species have been confirmed to be alternative hosts of Sss. Powdery scab is difficult to successfully control due to the pathogen’s ability to form resting spores and the scarcity of resistant cultivars. No single control method can completely control Sss, however, an integrated management approach is advised for management of Sss.
Pest/Pathogen of the Month: August
Scientific name: Sclerotinia sclerotiorum (Lib) de Bary.
Common names: white mold, cottony root, watery soft rot, stem rot, drop, crown rot and blossom blight.
Sclerotinia sclerotiorum (Lib.) de Bary is an Ascomycete fungus in the order Helotiales. This species is a multi-host necrotroph that infects more than 400 plant species, with lettuce, sunflower, canola and sugar bean being of economic importance to the South African agriculture industry. Sclerotinia sclerotiorum is characterized by the production of long-term survival structures called sclerotia which form an important part of the infection cycle. These structures can lay dormant in the soil for up to 8 years until environmental conditions become conducive for germination. Carpogenic germination of sclerotia results in the formation of a sexual structure (the apothecia) at soil level, which releases millions of ascospores that start the infection cycle. Myceliogenic germination forms hyphae and mycelium that results in direct sub-terrain infection of host plants, although this is limited to a radius of about 2 cm around the sclerotia. Signs and symptoms of disease depend on the host plant infected and can include water-soaked lesions or dry lesions on stems, leaves, fruits, or petioles. The presence of white fluffy hyphae on the host surface during high humidity forms the basis of the name “white mold” that refers to the disease.
Photograph:Sclerotia (black structures) mixed in with soybean (Photo by Lisa Rothmann, taken from The South African Sclerotinia Research Network (SASRN) website)
Pest/Pathogen of the Month: July
Scientific name: Teratosphaeria destructans
Teratosphaeria destructans is one of the most devastating pathogens to Eucalyptus forestry. It causes severe shoot and leaf blight on young Eucalyptus plantation trees and was, until recently, only known from South East Asia. In South Africa, T. destructans was first discovered in the KwaZulu-Natal Province by the FABI team in 2015. It has subsequently also spread to plantations of E. grandis and its hybrids in other sub-tropical parts of the country. Some good news is that the South African T. destructans population consists of a single genotype and mating type. This indicates that sexual reproduction is currently not possible and that the pathogen most likely entered South Africa as a single introduction. Care should be taken not to introduce the opposite mating type or additional genotypes.
Photo credit: https://www.fabinet.up.ac.za/index.php/news-item?id=274
Pest/Pathogen of the Month: June
Scientific name: Rosellinia necatrix
Common names: White root root
Rosellinia necatrix Berl. ex Prill. is the causal agent of white root rot on various plant species, including almond, apple, peach, orange, pear, grape, coffee, olive and avocado. The genus Rosellinia consists of multiple species capable of causing disease, however, R. necatrix is the most widely distributed and devastating. R. necatrix was first identified in South Africa in 1974 on apple and pear trees in the Western Cape. It is an ascomycete, saprophytic pathogen that causes rotting of the roots and collapse of host conducting vessels leading to wilting and death. This fungus is soilborne and can survive in the soil on woody debris and organic matter for long periods of time. White root rot is difficult to diagnose since foliar and root symptoms are unspecific, therefore, the disease is often mistaken for Phytophthora root rot. Some hosts do not show any foliar symptoms until the plant suddenly dies, sometimes with fruit and leaves still attached to the tree. A distinguishing symptom is the presence of white mycelial growth on the root surface, in the soil and underneath/on top of the bark at the crown of the tree. Disease control options are limited due to the pathogen’s hardy resting structures, extensive soil penetration and ability to withstand drought, acidic soils and many common fungicides.
Pest/Pathogen of the Month: May
Scientific name: Pectobacterium
Common names: Soft rot bacteria; Soft Rot Enterobacteriaceace
Pectobacterium species, formerly known as Erwinia species, are the main causal agents of soft rot, blackleg and aerial stem rot of potatoes as well as of many other vegetables and ornamentals (Figure 1). The genus consists of many species capable of causing disease. It is a Gram-negative, opportunistic pathogen that causes tissue maceration through the production of pectinolytic enzymes that result in cell wall degradation. The pathogen is mainly spread by latently infected propagation material, although it can also be spread by contaminated irrigation water, equipment and insects. It isn’t soilborne and therefore doesn’t overwinter or survive in the soil for longer than six months in the absence of a host. The pathogen remains latent within the plant until favourable environmental conditions cause a shift from latency to disease development. Symptom expression is dependent on quorum sensing and therefore, pectinolytic enzyme production only starts after the pathogen population reaches a critical threshold. The main environmental factor that promotes disease development is high soil moisture, which creates an anaerobic environment, favouring the growth of this facultative anaerobe.
Pest/Pathogen of the Month: April
Scientific name: Bathycoelia distincta (old name: B. natalicola - distant)
Common names: Two-spotted stink bugs
Bathycoelia distincta is a Heteropteran that was discovered in 1984 in the Limpopo region (South Africa). This indigenous species belongs to the Pentatomidae family, well-known to comprise the highest number of economically important species in the world. These bugs are phytophagous and feed by inserting their stylets (mouthpart) into the food source to suck up nutrients, resulting in potential damage to plant tissue. Pentatomidae species are also characterized by their general ovoid form, antennae, and tarsi segmented into five and three parts respectively, and a short triangular scutellum. They are commonly called stink bugs, due to the odour produced by their scent glands. The Pentatomidae is the largest family within the superfamily Pentatomoidea (Heteroptera suborder, true bugs) and comprises of approximately 5000 of the estimated 8000 species, followed by the Lygaeidae, Reduviidae and Miridae. Species within Pentatomidae are further divided into eight subfamilies namely: Asopinae, Cyrtocorinae, Discocephalinae, Edessinae, Phyllocephalinae, Podopinae, Serbaninae and Pentatominae (the largest), in which B. distincta belongs.
Pest/Pathogen of the Month: March
Scientific name: Pseudomonas syringae
I assure you, there is nothing fake about Pseudomonas syringae, so named after its host of origin, Syringa vulgaris commonly known as lilac, or its devastating effect on agricultural systems. P. syringae is listed as #1 in the top 10 most economically significant plant pathogenic bacteria1. This bacterium is known as a species complex that comprises nine genomospecies (DNA-DNA hybridization delimitation), 13 phylogenetic groups (sequence-based similarity of four housekeeping loci), and 19 phylogenomic groups (whole-genome based phylogeny). There are currently over 60 pathovars (strains characterized based on host of origin or similarity of symptoms caused). P. syringae is widely used as model pathogen to study bacterial pathogenesis, molecular mechanisms of plant-microbe interactions, microbial ecology, as well as evolution and epidemiology.
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