PPNs of eggplant in Egypt
Root-knot nematodes (Meloidogyne spp.)
Nematode spread and distribution patterns
These endoparasites have obligate feeding habit on crop roots. The nematode group is characterized by its broad host range that shackles the accessibility of resistant/immune cultivars in crop sequences. Such a wide range covers a wide variety of plants; not only woody and herbaceous plants, but generally monocotyledonous and dicotyledonous plants. Eggplants grafted on RKN-resistant rootstocks are sometimes used. Hence, nematode-resistant sources are still necessary for breeding programs given the fact that some related Solanum species such as S. torvum has high resistance level to Meloidogyne spp. (Daunay 2008; Öçal and Devran 2019; Gisbert et al. 2011). Also, eggplants were genetically engineered via the tomato Mi-1.2 gene and thus acquired resistance to M. javanica (Goggin et al. 2006). Papolu et al. (2016) suggested that cystatins can help in enhancing the eggplant resistance within a strategy that could eventually increase crop yield. Although this nematode group includes more than 90 species, only four species (M. incognita, M. hapla, M. javanica, and M. arenaria) are globally main pests; other seven species are serious from a regional or local scope (Moens et al. 2009).
Meloidogyne spp. perform a major obstacle especially for horizontal expansion of crop production including eggplant cultivation in Egypt, because these species thrive in light and sandy soils. Hence, reclaimed deserts present an optimum status for their establishment and multiplication (Abd-Elgawad 2020a). Consequently, RKNs are the most widespread and significant PPN genus damaging many plant species in Egypt. This genus recorded 62.5% frequency of occurrence (Ibrahim et al. 2010; Abd-Elgawad 2019a). Its species were detected in as many as 96.26% of the examined fields in such lands (Bakr et al. 2011). The fields are perceivably planted to field and horticultural crops, e.g., eggplant. This study covered various categories of reclaimed but light soils located at El-Beheira (El-Tahrir), Minufiya (El-Sadat), and Sharkiya (El-Salhiya) governorates. Also, RKNs were more dominant in agricultural soil of Sahl El-Teina, Beer El-Abd, and El-Sheikh Zowaiid with the percentage frequency of 27.6, 48.1, and 33.3%, respectively (Korayem et al. 2014). Clearly, densities of RKN population differed from one field to another as the nematodes were subjected to various control methods and production practices, in addition to biological and physical factors.
Frequently, PPNs display clumped distribution in Egypt (Abd-Elgawad 1992; Abd-Elgawad and Hasabo 1995; Abd-Elgawad et al. 2016, a, b, c) and worldwide (Duncan and Phillips 2009; Abd-Elgawad and Askary 2015) in general. Therefore, patches of RKN infections possibly in scattered areas/sites of the field can be observed where eggplant seedlings fail to grow normally. So, weakening symptoms on stunted plants often occur in spots of non-even plant growth, i.e., non-uniform damage of eggplants over the whole planted area (Fig. 1). Nematodes spreading in these spots or patchy areas should be adequately managed at the season-start. If not, they may expand and develop in size of the infested field sites to cover almost all eggplant-cultivated fields.
RKN Pathology and losses
Like other PPNs, the common symptoms of RKN injury on eggplant are stunted plants and decrease in their growth parameters which ultimately lead to considerable loss. The intensity of RKN disease symptoms is positively correlated with initial density of RKN populations and their reproduction capacity which depend on the biological and environmental factors during the plant growing season. Eggplants infected by RKNs are often more deteriorated by noxious weeds than plants without RKN infection. This is definitely because the plants cannot often stand for weed competition or any other stresses. Factually, plant weakness of infected plants is mostly due to insufficient mineral nutrition or water supply to the tops. Hence, the infected eggplants exhibit symptoms that are characteristic of nutrient deficiency such as leaf yellowing/chlorosis, slow response to adequate soil moisture conditions, and premature wilting. Unmistakably, RKNs infection and consequent feeding on plant roots will result in knot-like swellings known as nematode galls on the eggplant roots (Fig. 2) due to formation of giant cells inside the tissues of plant roots. When plants are harshly attacked and infected by RKNs, the intact root system is diminished to a confined volume of remarkably RKN-galled roots where the vascular system of the eggplant roots is greatly disorganized. Factually, eggplant growth reduction and loss in yield are correlated with nematode population density, the magnitude of host suitability, and the implemented control measures (e.g., Abd-Elgawad et al. 2019), as well as biological and ecological conditions. Eggplant roots may bear different numbers and sizes of RKN galls with different shapes and locations on the root system (Fig. 2).
Action thresholds necessitate RKN control for infected eggplants if a single nematode was detected per 100 cm3 of eggplant-cultivated soil in Egypt and other countries (Abd-Elgawad and Askary 2015). Therefore, promising research results especially of RKN biocontrol on eggplant which will be presented herein should further be practiced on wider scale in Egypt.
Lesion nematodes (Pratylenchus spp.)
Pathogenicity
In fact, the losses and prevailing distribution of RKNs often hide the role and losses of other PPN genera such as lesion nematodes, Pratylenchus, and reniform nematode, Rotylenchulus on eggplant. Clearly, Pratylenchus spp. have a broad host range feeding on most cultivated Various weed species also may back the nematode reproduction as suitable hosts. Admittedly, different species of Pratylenchus are associated with various crops, grasses, and weeds in Egyptian soils (Abd-Elgawad 2019a). Hence, it is quite possible to cultivate eggplant in the above-mentioned soils reported by Ibrahim (2006) to be infested with one or more of Pratylenchus spp. However, little information is available on the exact eggplant yield damage/loss by a definite Pratylenchus sp. in Egypt. Moreover, lesion nematodes can also predispose plants to invasion by some genera of soil borne pathogens in addition to their direct damage in parasitizing eggplants. For example, in steam-sterilized soil inoculated with Verticillium alboatrum and Pratylenchus penetrans, McKeen and Mountain (1960) demonstrated a synergistic relationship between V. alboatrum and P. penetrans. At low and intermediate levels of Verticillium inoculum, the nematode increased wilt may be a function of the number of nematodes present. Except at the high inoculum levels of Verticillium, significantly larger numbers of P. penetrans occurred in eggplant roots in the presence than in the absence of the fungus. On the other hand, Mizukubo (1995) tested the reproduction of three populations of Pratylenchus coffeae in Japan and proved the presence of physiological races among them on eggplant. Also, Di Vito et al. (2002) examined six population of Pratylenchus spp. from the Mediterranean region and found that eggplant was good host for P. neglectus but P. pinguicaudatus reproduced better on faba bean, lentil and pea than on eggplant. Machado and Inomoto (2001) used the nematode-reproduction factor or Rf [= the nematode-final population (Pf) divided by the nematode-initial population (Pi); or Rf = Pf/Pi] to examine host suitability of eggplants to Pratylenchus brachyurus. They reported that intermediate nematode populations occurred at 65 days after P. brachyurus inoculation (Pf/Pi = 2.78), but in Khan’s (1992) study of the same crop, a strong population increase (Pf/Pi = 31.4 at 75 days after inoculation) was demonstrated.
Control measures
Pratylenchus spp. should have a comprehensive control measures within integrated pest management (IPM) approach. A basic control measure in this approach is to use nursery stock free from the nematodes. Also, rotating resistant plant cultivars/species such as pearl millet with susceptible eggplants could be utilized successfully in Egypt. Like other solanaceous crops such as tomato and pepper, chemical nematicides can effectively control PPNs on eggplant but their related health hazards, development of pest resistance, and environmental pollution should be avoided. Also, raising awareness of growers via guiding them to sound production exercises such as destruction of weeds and infected-plant residues in eggplant fields can upgrade nematode control (Abd-Elgawad 2019a, b).
Reniform nematode (Rotylenchulus reniformis)
Lone et al. (2014) stressed that Rotylenchulus with 10 species are prevalent and significant group of PPNs globally but often detected in tropical and subtropical climate. Among its species, R. reniformis with at least 314 plant host species is the most prominent and economically appreciated one (Robinson et al. 1997). Reniform nematodes rob nutrients of infected plants and reduce the quantity and quality of the eggplant fruits. Symptoms are primary expressed as minimized root systems, nutrient deficiency, and stunting of the infected plants. Reduction in plant longevity negatively affects harvest duration. Economically appreciated management of reniform nematodes in view of their worldwide distribution and broad host range especially for R. reniformis has always been a dire need for growers globally (Lone et al. 2014).
The lifecycle of reniform nematode was recently reported (Abd-Elgawad 2020a). Its time span is frequently less than three weeks in warm regions. The nematode can survive for more than two years without its host under stressed condition, i.e., in dry soil. In such a case, R. reniformis can live in a dormant state stimulated by drought and the nematode is almost dehydrated with diminished metabolic activity; a condition called anhydrobiosis that helps R. reniformis to live without water for long time (Wang 2019). On infecting plant roots, the nematode female establishes a feeding site comprising syncytial cells.
Most manifestations and disease signs resulting from PPN infection are generally identical to those of lack in nutrients and eggplant irrigation. As R. reniformis feeds on the eggplant roots, the growth of the whole plant is decreased. Additional infection by other soil borne pathogens after R. reniformis establishment on eggplant roots can worsen plant status. Both R. reniformis and RKN have sexual dimorphism.
Other plant-parasitic nematodes
Additional PPNs have obviously been less recognized regarding their economic significance (Ibrahim 2006) and are entitled to more investigations in Egypt. These include the nematodes in the genera Xiphinema, Longidorus, Hoplolaimus, Paratrichodorus, Trichodorus, and Tylenchorhynchus. They are frequently associated with eggplant roots in Egypt but in low frequency of occurrence and population densities. Therefore, further investigations on any of them may reveal whether they demonstrate economic importance and determine their damage on eggplants in Egypt.
Also, the significance of other nematodes on eggplants has been reported from other countries, e.g., Belonolaimus longicaudatus in Florida, USA (Noling 2019). Likewise, action thresholds are suggested for any detection of sting or stubby-root nematodes; usually in sample size of 100 cm3 soil of eggplant-planted field. The corresponding thresholds are 10, 40, and 80 nematodes for awl, stunt, and lesion nematodes, respectively (Abd-Elgawad and Askary 2015). Yet, further PPN species and additional relevant factors may alter the threshold from one location/season to another (Wang 2019).