Helianthus sp.

D. Howland, Amanda & Quintanilla, Marisol, 2023, Plant-Parasitic Nematodes and their Effects on Ornamental Plants: A Review, Journal of Nematology 55 (1), pp. 1-13 : 5-8

Helianthus sp. Sunflower AphelenChOides ritzemabOsi; MelOidOgyne Khan, 2015 ;

inCOgnita ; ParatylenChus prOjeCtus Jenkins, 1956 ; Kohl, 2011; Loof, 1975;

PratylenChus thOrnei Rashad et al., 2011

Zinnia sp. Zinnia AphelenChOides besseyi , A. fragariae, Khan, 2015 ; A. ritzemabOsi, MelOidOgyne spp. Kohl, 2011;

Wheeler et al., 2018

Jagdale and Grewal, 2006; Kohl, 2011; Mitiku, 2018). The three main species of economic importance to the ornamental industry are A. fragariae (Ritzema Bos, 1891) Christie, 1942 , A. ritzemabOsi (Schwartz, 1911) Steiner and Buhrer, 1932, and A. besseyi Christie, 1942 ( Kohl, 2011). Foliar nematodes are primarily endoparasites that feed predominately inside the leaves, but depending on environmental conditions, foliar nematodes can also feed externally on the leaves and flower buds of some plants. These nematodes mostly overwinter in the soil and migrate up the outside of plant stems in films of water to the leaves. They are spread easily through infected plant seeds and are commonly splashed around in water via rain or overhead irrigation from infected plants to plants nearby. Symptoms from foliar nematode feeding are chlorotic, angular lesions on the leaves that run parallel to major leaf veins and can eventually become necrotic if feeding persists ( Figs. 2C,D ). This can lead to leaves having a dry, tattered appearance, and severe infection can kill the whole leaf, causing defoliation ( Jagdale and Grewal, 2006; Kohl, 2011; Mitiku, 2018; Phani et al., 2021).

Management of foliar nematodes can be difficult since AphelenChOides spp. can survive in infested dried leaf debris and in dormant plant crowns for many years, and they are easily splashed around ( Jagdale and Grewal, 2006; El-Saadony et al., 2021). However, there are various methods that can be employed to manage these nematodes. Current management methods include using drip irrigation instead of overhead watering and sanitation of tools, pots, and soil that come in contact with infected plants by heat treating them via baking or steaming ( Mitiku, 2018). Other treatments include hot water drenches on infected plants, their leaves, and dormant plant materials ( Jagdale and Grewal, 2004; Kohl, 2011; Mitiku, 2018); the use of humic acid derived from manure ( Nagachandrabose and Baidoo, 2021); and plant resistance in some plant species ( Kohl, 2011; Mitiku, 2018; El-Saadony et al., 2021).

Virus veCtOring nematOdes: There are two different types of nematode transmitted viruses based on the viruses’ particle shape: NEPO viruses (nematode polyhedral viruses) and TOBRA viruses (tubular or rod-shaped viruses) ( Taylor, 1972). These viruses are transmitted through nematode feeding on plant tissues. NEPO viruses are transmitted by Xiphinema (dagger) and LOngidOrus (needle) nematodes; both ectoparasitic nematodes are large and slender. Their host range consists of perennial and woody plants, grapevines, orchids, and small fruits ( Taylor, 1972; Handoo, 1998). Main NEPO viruses for ornamental plants, such as geraniums, petunia, and tulips, are tomato ringspot virus (ToRSV), tobacco ringspot virus ( TRSV), Arabis mosaic virus (ArMV), and tomato black ring ( TBRV) virus ( Engelmann and Hamacher, 2008). While there are numerous NEPO viruses, most of which are species dependent, general symptoms include pronounced ring and mosaic lined patterns, chlorotic flecking, mottling, leaf curling and necrosis, stunted plants, and general plant decline.

TOBRA viruses are transmitted by TriChOdOrus and ParatriChOdOrus species (stubby root nematodes), which are thick nematodes with a curved stylet. Similar to dagger and needle nematodes, they feed externally at the root tip, causing the root tips to swell and become stubby ( Taylor, 1972). The main viruses are Tobacco mosaic virus ( TMV), Tobacco rattle virus ( TRV), Pea early-browning virus ( PEBV), and Pepper ringspot virus (PepRSV) which can affect plants like petunias, tulips, and lilies ( Taylor, 1972; Handoo, 1998; Engelmann and Hamacher, 2008; Macfarlane, 2010; Khan, 2015). General symptoms of TOBRA viruses include chlorotic and necrotic spots or strips on leaves, mosaic, mottling, light or dark colored flecks on flower petals, oval lesions, flower malformation, and stunted plants ( Engelmann and Hamacher, 2008; Madhavan et al., 2021).

Management of these nematodes to prevent virus spread and transmission include using only virus-free planting material, hygienic measures and disinfection of tools and equipment, thermotherapy, destroying virus infected plants to help prevent the spread of inoculum, and regular testing of plant stocks ( Taylor, 1972; Engelmann and Hamacher, 2008; Adaskaveg and Caprile, 2009; Madhavan et al., 2021). Weed control is especially important since they can serve as alternative hosts for nematodes and can act as virus reservoirs. For example, dandelions, TaraxaCum OffiCinale , and other broadleaf weeds can harbor viruses, and nematodes can feed on them, obtain the virus, and then transmit it to ornamental plants in the same field ( Adaskaveg and Caprile, 2009). Fumigants for controlling the nematode vector can be used preplanting as a management option. Additionally, using both nematode and virus resistant cultivars can be a very effective management option. For example, in tulips, there are several TBV- resistant cultivars such as ‘Cantata’, ‘Juan’, ‘Madame Lefeber’, and ‘Princeps’ ( Madhavan et al., 2021). While these management methods can be effective, there is a need for more research, especially since plant propagules can also harbor viruses leading to their spread.

Other plant-parasitiC nematOdes: There are many other PPNs that can infect and severely damage ornamental plants ( Table 1). Some of the other main PPNs are PratylenChus spp. , or lesion nematode, which is a migratory endoparasite that has a host range of over 400 plant species, including amaryllis ( Christie and Birchfield, 1958; Nong and Weber, 1965), ferns ( Rhoades, 1968), and foliage plants such as rubber plants ( Pinochet and Duarte, 1986). Symptoms of lesion nematode feeding in ornamental plants has been reported to be stunting of roots and shoots, chlorotic foliage, foliage discoloration, wilting, root swelling, brown lesions, destruction of the entire root systems, and severe yield loss ( Christie and Birchfield, 1958; Rhoades, 1968; Mitiku, 2018). Symptoms can also include necrotic lesions on plant roots which can lead to secondary infections from bacteria and fungi causing disease complexes. RadOphOlus similis (Cobb, 1893) Thorne, 1949 , the burrowing nematode, is another migratory endoparasite that is an important pest in anthurium and black pepper ( Haegeman et al., 2010; Khan, 2015), with over 350 additional hosts. Feeding from R. similis in the roots and stems causes root decay and rot, severe plant stunting and chlorosis, and can cause plant dieback and death ( Borgohain, 2016; Wang et al., 2016). In anthurium rhizomes, R. similis can also disrupt the vascular bundles causing necrosis and anatomical alterations in the roots (Vovlas et al., 2003), or they can fail to produce any noticeable symptoms in anthurium canes, which are used to plant new fields, leading to the accidental spread of this nematode ( Sipes and Myers, 2018).

ROtylenChulus renifOrmis Linford and Oliveira, 1940 , the reniform nematode, is a semiendoparasitic nematode that partially penetrates plant roots. These nematodes produce no plant symptoms in some ornamental plants like daylily ( Inserra et al., 1998), but they can be devasting parasites to crops such as cotton ( Koenning et al., 2004). HeliCOtylenChus spp. , the spiral nematode, is a migratory ectoparasite that has a large host range, but very little research has been conducted on this nematode in ornamental plants, except for ROtylenChus buxOphilus Golden, 1956 , the boxwood spiral nematode, which causes slow decline in boxwood plants ( Lehman 1984; Eisenback, 2018). Another migratory ectoparasite, ParatylenChus spp. , or pin nematode, is also commonly found in ornamental plant fields, such as daylily ( Howland et al., 2022), but to date, very little research has been conducted on this nematode species in ornamental plants except on P. shenzhenensis Wang, 2013 in anthurium (Wang et al., 2016). Symptoms of ParatylenChus spp. infection includes stunting, low quality plants, and decreased yields.

Figures

Figure 2: Light micrograph of an adult male (A) and head (B) of AphelenChOides spp. extracted from HeliOpsis spp. leaves. Angular lesions (C,D) on the leaves of two varieties of HeliOpsis spp. infected with AphelenChOides spp.