Foliar nematodes, primarily from genera such as Aphelenchoides, Ditylenchus, Anguina and Litylenchus, are significant plant pathogens affecting ornamental, agricultural and forest plants through leaf infection, leading to characteristic lesions and potential crop loss. These nematodes exhibit diverse symptoms including vein-delimited lesions and necrosis and can survive in various environmental conditions, including dry tissues and decaying material. Recent studies have advanced understanding of their distribution, ecology and detection methods, including molecular diagnostics like PCR and innovative rapid assays such as LF-RPA. Climate modeling tools like MaxEnt have effectively predicted potential geographic distributions, aiding in management planning. Control strategies emphasize removal of infected plants, proper sanitation, chemical treatments and cultural practices to limit spread. Emerging chemical nematicides further enhance management options, highlighting the importance of integrated approaches to mitigate foliar nematode impact. This review highlights the Current status, Disease and Symptoms, Importance and management strategies associated with Foliar nematodes.

Aphelenchoides, Anguina, Ditylenchus, Litylenchus.

Nematodes likely evolved around 400 million years ago, predating the Cambrian explosion. These unsegmented, bilaterally symmetrical, triploblastic animals have a pseudocoelomic body cavity. Key damaging genera include Meloidogyne, Rotylenchulus, Heterodera, Radopholus, Pratylenchus, Globodera, Aphelenchoides and Tylenchulus (Anamika et al., 2023).

Foliar nematodes are mostly pathogens of ornamental crops in greenhouses, nurseries and forest trees, as well as field crops (Kohl, 2011). Foliar nematodes include several nematode genera among which Aphelenchoides, Anguina, Ditylenchus and Litylenchus (Kohl, 2011). Foliar nematode Anguina tritici, it was a first plant parasitic nematode reported in year 1743 (El-Saadony et al., 2021). Foliar nematodes have been documented as associated with more than 1100 different species of plants, belonging to 126 botanical families, to include dicots, monocots, gymnosperms and angiosperms, ferns and mosses (Kohl, 2011). Aphelenchoides, as well as nematodes of genus Litylenchus, are phytoparasites known to infect leaves, stems and buds (Sánchez Monge et al., 2015). The damage caused by the foliar nematodes can cause marketability problems in ornamentals because they interfere with the appearance of the plant or they can reduce yield in food crops (Kohl, 2011).

They infect plant leaves by migrating upward along the stems and entering through the leaf stomata. Since their movement depends on water, infection is more likely when plant surfaces are recently moistened by dew, rain or overhead watering. Foliar nematodes feed primarily from within plant tissues (endo-parasitically) and less frequently from outside (ecto-parasitically), depending on the environment and the type of host plant. When feeding inside leaves, they cause lesions that are brown, black or chlorotic, often outlined by veins; these lesions can become necrotic as they age (Kohl, 2011). 

MAJOR GENERA AND SPECIES OF AERIAL NEMATODES 

The primary aerial nematodes of significance in plant pathology encompass several key genera and species. It include Aphelenchoides, Anguina, Ditylenchus and Litylenchus (Kohl, 2011). Aphelenchoides besseyi is known for causing white tip disease in rice and is also capable of infecting strawberries and ornamental plants. Additionally, Aphelenchoides fragariae and A. ritzemabosi are responsible for foliar necrosis in strawberries as well as various ornamental species (Desaeger and Noling 2017). Ditylenchus dipsaci stands out as a versatile pathogen, infecting a broad range of hosts and serving as a quarantine pest in numerous countries (Subbotin et al., 2005). These nematode species have evolved adaptations that allow them to survive in dry plant tissues, seed stocks and moist foliar environments, facilitating their persistence and spread across different agricultural and ornamental settings. 

SYMPTOMS AND DISEASE IMPACT 

Damage caused by foliar nematodes can resemble injuries resulting from bacteria, fungi, viruses, nutrient deficiencies or chemical exposure. Additionally, nematodes may interact with specific fungi or bacteria, leading to severe foliar blight. In strawberries, damage symptoms caused by mites or thrips can also appear similar to those caused by nematodes (Desaeger and Noling 2017).

Foliar nematodes occur mainly within the leaves and crowns of their hosts, where infested buds, young leaves or shoots typically become deformed and will not develop properly. Plants will often remain undersized, become bushy or distorted and produce little or no marketable foliage or flowers. In strawberry, young plants are often dwarfed and present a spider-like appearance. Leaves appear crimped, twisted or crinkled and are a darker green than normal. A reddish color may appear on the leaf edges and veins and leaves tend to become more brittle, with tips of young leaves and stipules of older plants drying up and turning brown. Petioles are often short, thick, less pubescent than those of unaffected plants and have a reddish cast. Damaged plants put on fruit late and it is often of inferior quality, sometimes showing "phyllody" or "broccoli fruit" (Desaeger and Noling 2017).

Xu et al. (2020) studied on Population structure and species delimitation of rice white tip nematode, Aphelenchoides besseyi, in China. Early symptoms include yellowing and wrinkling of new leaf sheaths, with infested leaves showing white splash patterns or yellow patches; edges may be deformed. Severe infections cause kinked, shortened flag leaves, small distorted grains and sterile clusters on upper tillers.

Sturhan and Brzeski (2020) Studied on Stem and bulb nematodes, Ditylenchus spp. Infected plants show yellowing, wilting and premature death, with distinct patches of discolored plants. Garlic bulbs turn brown, shrivel, crack and develop swollen, blister-like areas. Onion bulbs become soft, brown and rot internally, with damage worsening during storage.

Among this foliar nematode Anguina tritici, commonly known as the wheat seed gall nematode. It is responsible for causing ear-cockle disease. It was the first plant-parasitic nematode to be documented in scientific literature, dating back to 1743. Its hosts encompass wheat, triticale, rye and other related grasses, with wheat being its primary host (El-Saadony et al., 2021)

Subbotin (2024) studied the nematode (Aphelenchoides fragariae) targets the above-ground parts of the plant, leading to deformities such as twisted shoots, small puckered leaves with crinkled edges, reddened petioles and discolored areas that are hard and rough in texture. These damage symptoms can be mistaken for powdery mildew or bacterial infections. Specifically, Aphelenchoides fragariae can significantly reduce strawberry yields, as heavily infested plants often fail to grow properly or produce fruit.

STATUS

Holajjer et al. (2022) surveyed the Prediction of potential geographic distribution of exotic nematode, Aphelenchoides fragariae in India based on MaxEnt ecological niche modelling. The MaxEnt model effectively predicted climate suitability for A. fragariae in India, with AUC values of 0.972 for training data and 0.942 for testing data. Predicted probabilities of presence, ranging from 0 to 1, were categorized into suitability levels: highly suitable (0.8-1.0), optimally suitable (0.6-0.8), moderately suitable (0.4-0.6), marginally suitable (0.2-0.4) and unsuitable (0-0.2). Highly suitable areas are mainly in northern and eastern India, including Himachal Pradesh, Uttarakhand, Arunachal Pradesh and parts of Jammu and Kashmir, with probability scores of 0.79 to 1.0. Optimally suitable regions also cover northern and eastern India, such as Meghalaya, Mizoram and some high-altitude areas in central and southern India, with scores between 0.59 and 0.79. Areas with scores of 0.39-0.59 and 0.19-0.39 were classified as moderately and marginally suitable, respectively, for the establishment of A. fragariae. 

Munawar et al. (2023) studied New Records of Ditylenchus Species from Southern Alberta, Canada. This study identified four Ditylenchus species in southern Alberta, Canada, including D. anchilisposomus, D. clarus, D. tenuidens and D. valveus three of which are new records for Canada. Morphological and molecular analyses confirmed their identities and phylogenetic relationships.

Oliveira et al. (2024) surveyed foliar nematodes in Florida strawberry fields from 2016 to 2021, identifying Aphelenchoides besseyi only in early years on healthy leaves, while A. bicaudatus (new to Florida) and A. rutgersi were found on declining leaves. Morphological and molecular analyses confirmed their identities and global relationships. Both species were absent from healthy leaves, suggesting they feed on fungi rather than plants in natural conditions. However, A. bicaudatus showed limited ability to damage soybean leaves under artificial conditions. This study improves knowledge of foliar nematode diversity and ecology, highlighting the need for combined approaches for effective nematode management.

IMPORTANCE

A. tritici is on the US Pest List of Economic and Environmental Importance and 'Pests Lists' for Argentina, Brazil, Chile, Colombia, Ecuador, Egypt, Guatemala, Indonesia, Israel, Madagascar, Namibia, Nepal, New Zealand, Paraguay, Peru, South Africa, Taiwan, Thailand, East Timor, and Uruguay (EPPO, 2020). 

Howland and Quintanilla (2023) discussed the severe impact of plant-parasitic nematodes, including foliar types on ornamental crops such as chrysanthemum, hosta and fern. In these crops, symptoms like leaf blight and tissue collapse caused by foliar nematodes drastically reduce plant quality. Since ornamental plants are traded widely, infected materials pose a serious threat to both domestic and international markets with potential quarantine implications and trade restrictions.

Burke et al. (2024) studied the Beech leaf disease (BLD) is an emerging threat to American beech (Fagus grandifolia), spreading rapidly across the north eastern U.S. and into southern Canada. It causes leaf disfigurement, dark interveinal banding, curling and leaf drop, mainly affecting understory leaves and thinning the forest understory. Identifying BLD's cause is vital for protecting F. grandifolia and forest health. While the foliar nematode Litylenchus crenatae sub sp. mccannii is confirmed as a key factor, the role of other organisms remains unknown. This study examined leaf-associated fungi and bacteria to explore their potential contribution to BLD. Nematodes were detected at low levels in areas showing no symptoms, indicating that they can exist without causing visible signs of infestation.

Albaqami et al. (2024) emphasized the global concern regarding phytoparasitic foliar nematodes, noting their role in causing crop damage in leafy vegetables and herbs. The study reviewed sustainable approaches to combat these nematodes, highlighting the need for effective, eco-friendly control methods. In many developing countries, the lack of awareness and access to nematode diagnostics further aggravates the problem, leading to undetected spread and increasing economic burden on smallholder farmers.

DETECTION AND DIAGNOSIS 

A PCR assay using species-specific primers successfully identified Aphelenchoides fragariae from samples initially morphologically identified on Andrographis paniculata ('bitter king') in Indonesia. All isolates produced a 169-bp amplicon from the ITS1 rDNA region, confirming the species. Phylogenetic analysis showed clustering with global A. fragariae sequences. This represents the first molecular report of A. fragariae on bitter king in Indonesia and globally (Djiwanti and Miftakhurohmah 2022).

Han et al. (2024) developed a species-specific PCR assay for rapid, accurate detection of Ditylenchus destructor and D. dipsaci, overcoming limitations of traditional morphology. Targeting rDNA-ITS sequences, it offers high sensitivity, detecting single juveniles and low DNA levels and effectively identifies multiple haplotypes directly in plant roots and soil. This method enhances early diagnosis and improves pest management strategies.

Subbotin (2024) performed A rapid, reliable diagnostic method for Aphelenchoides fragariae using a lateral flow recombinase polymerase amplification (LF-RPA) assay targeting the ITS rRNA gene. This assay detects the nematode directly from crude extracts or plant tissues without DNA extraction, delivering results in 18-25 minutes with high sensitivity. Validated across various samples, it successfully identified A. fragariae in California seed samples and offers a practical tool for nematode diagnosis with minimal equipment.

MANAGEMENT STRATEGY

Current management approaches include the use of hot water treatments (drenches and dips) and preplant fumigation. Hot water drenches are applied to disinfect plants from pests such as insects and plant-parasitic nematodes (PPNs), including species like the stem and bulb nematode Ditylenchus dipsaci as well as A. fragariae and R. similis. Typically, plant materials such as bare-rooted plants, tubers, runners and dormant plant parts are treated by immersing them in large tanks of hot water followed by cooling in secondary water tanks to complete the process (Howland and Quintanilla 2023). 

The most crucial initial step in managing foliar nematodes is to eliminate infected plants. It's important not to place these infected plants in compost or cull piles, as foliar nematodes can survive drying out and could be reintroduced into the production area. Only pest-free planting material should be used and efforts should be made to prevent foliar nematodes from contaminating propagation zones. Avoid using infected cuttings or stock plants during vegetative propagation, as this is a key method of control. Additionally, reducing overhead watering and minimizing leaf wetness through proper plant spacing and placement can help manage the spread. Keeping greenhouses and propagation houses free of weeds is also essential, as many weeds can serve as hosts. Lastly, it's important to familiarize yourself with the characteristic symptoms of foliar nematode infection; to confirm their presence, samples should be taken and sent to a plant diagnostic laboratory for analysis (Joan et al., 2024). 

New chemical nematicides have been granted registrations in recent years like Fluopyram (Velum Prime®) and fluensulfone (Nimitz®) (Walia, 2024).

There is a lack of comprehensive data on the economic losses caused by these pathogens. Our understanding of their ecology and survival strategies remains limited. Many crops lack resistant germplasm, making control more challenging. There is a need for improved diagnostic tools that allow rapid detection directly in the field.  

Future research should prioritize genomics-based identification methods, studies on host-pathogen interactions and the development of environmentally sustainable management strategies.

Prajapati P.G., Patel M.I., Kansara S.S., Chopda G.B., Ghevariya T.V. and Patro S.P.  (2025). A Review on Status and Importance of Foliar Nematode in Plant Pathology. Biological Forum, 17(7): 18-21.