Plant Parasitic Nematodes Associated with Citrus Trees and Reaction of Two Citrus Cultivars to Tylenchulus semipenetrans in Northern Egypt

Introduction

Citrus is one of the most important economic fruit crops in various regions of the world. So, it has received a great attention in agriculture and industry. In Egypt, citrus has great attention due to its importance for local consumption or exportation to the European countries. Plant diseases caused by soil borne pathogens such as nematode and /or fungi have been considered as major problems in agricultural production throughout the world (Abd-El Gawad et al., 2009). In Egypt, plant parasitic nematodes especially Meloidogyne spp., Patylenchus spp., Tylenchulus semipenetrans, and Xiphinema spp. constitute one of the most important nematode pest groups on many economic fruit crops (Ibrahim et al., 2010).

The nematode of the citric fruits probably infest more than 50% of the citrus production areas. The losses of the citrus yields caused by these nematodes were estimated to be ≈10% (van Gundy, 1984). Studies on the economic yield losses of citrus trees due to T. semipenetrans infection showed that these losses might reach up to 10% - 30%, depending on the level of nematode infection (Duncan and Cohn, 1990 and Verdego-Lucas and McKenry, 2004).

Nematode infection of the citrus feeder roots might increase the severity of the drought stress on the citrus trees, and decrease the ability of these roots to uptake water and minerals from the soil. Affected trees have an unthrifty appearance, fewer yields, and smaller fruits, compared to the non-infected trees (Duncan and Cohn, 1990).

The objective of the present work, survey study was to identify phytoparasitic nematodes associated with citrus trees in northern Egypt and estimate the frequency of occurrence and population densities of these nematodes in the collected rhizospheric soil samples and reaction of some citrus cultivars to T. semipenetrans.

MATERIALS and METHODS

1. Survey study

A total of 432 soil and root samples were collected from the rhizosphere of citrus trees showing disease symptoms during the period from 2017 to 2019 from different localities at Alexandria (142 samples), El – Behera (65 samples) and Kafr El-Sheikh (225 samples) governorates, northern Egypt. Soil and root samples were placed in polyethylene bags, and kept in the refrigerator at 4^ºC until nematode extraction within 48 hrs. Whenever processed, each sample was thoroughly mixed and rendered for nematode extraction using wet-sieving and Baermann-pan technique (Goodey, 1957; Ayoub, 1980). Plant parasitic nematodes in aliquots of 1 ml of each nematode suspension were placed in Peter’s 1 ml eelworm counting slide and the nematode genera were identified and counted with the aid of a compound microscope. The nematodes were identified to the generic level, based on the morphological characteristics of adult and larval forms following the descriptions of Thorne (1961), Goodey (1963) and Mai and Lyon (1975).

1. Reaction of some citrus species to Tylenchulus semipenetrans

The reaction of mandarine, Citrus deliciosa and lime, Citrus aurantifolia to T. semipenetrans was determined under greenhouse conditions. Citrus seedlings of five months old were planted in 20 cm diam. prickled polythene bags, containing 4 kg of steam-sterilized sandy clay soil (1:1, v/v) as one seedling/bag. Soil of each bag was infested with 5000 J₂s/ bag of T. semipenetrans the nematode inoculum was added in 10 ml water suspension into four holes around the seedling roots. Non inoculated plants served as control. Nematode treatments and control were replicated 5 times. Bags were arranged in a randomized complete block design on a greenhouse bench and irrigated and fertilized as needed.

Plants were harvested after 180 days of nematode inoculation and assessed for nematode infection. Root systems were carefully washed with a gentle stream of running tap water. Number of J₂s in each experimental unit (final nematode population =Pf) was determined and the nematode reproduction factor (Rf = Pf / Pi) was calculated to determine the host suitability. The tested citrus plants were rated according to their nematode Rf values, as follows: Plants with Rf = 0 were considered as resistant; those with Rf = 0.1-0.5 moderately resistant; those with Rf = 0.6-1.0 moderately susceptible; those with Rf =1.1-5.0 susceptible; and those with Rf > 5 were considered as highly susceptible (Ali, 2005). The fresh and dry weights of the shoot and root systems of the harvested plants were also determined.

Data were statistically analyzed using a computer statistically analysis system (SAS institute Inc., 2000). and means were separated using least significant differences test (LSD)at P ≤ 0.05.  

RESULTS and DISCUSSION

Data presented in Table 1 showed the frequency of occurrence (FO) and population density (PD) of plant-parasitic nematode (PPN) genera associating the citrus trees in Alexandria, El-Behera and Kafr El-Sheikh governorates.

Nine PPN genera were found in association with soil and roots of the surveyed citrus trees i.e. Tylenchulus semipenetrans, Trichodorus, Rotylenchus, Helicotylenchus, Aphelenchoides, Tylenchus, Pratylenchus, Tylenchorhynchus and Meloidogyne. The nematode genera; Helicotylenchus, Pratylenchus, Trichodorus and Tylenchulus semipenetrans (Table1). were detected in all the soil samples collected from citrus trees fields.

Soil samples from Alexandria governorate (Abees location) showed that four nematode genera were associating the rhizosphere of citrus trees including Tylenchulus semipenetrans which was the most frequent nematode genus detected with 100% FO. The nematode genera Tylenchorhynchus and Pratylenchus detected 18-20% FO followed by Trichodorus (FO= 11%). Tylenchulus semipenetrans showed the highest population densities (PD)(2695 nematodes /250g soil). On the other hand, the lowest Pd was recorded for Trichodorus (PD= 11 nematodes/250g soil) Table 1).

Soil samples of Alexandria governorate (El-maamora location) showed the presence of eight nematode genera in the rhizosphere of citrus trees. Tylenchulus semipenetrans was also the most prevalent with (FO= 100% and PD=1105 nematodes /250g soil) (Table 1).

Five nematode genera were detected in the collected citrus soil samples from El-Behera governorate (Abu Homus location) Tylenchulus semipenetrans was the most prevalent with (FO= 100% and PD= 6810 nematodes /250g soil).The nematode genera Helicotylenchus and Trichodorus had 10-20% FO. and PD of 6 -23 nematodes /250g soil) (Table 1).

Six nematode genera were detected in the collected citrus soil samples collected from El-Behera governorate (KafrEl-Dawar location) Tylenchulus semipenetrans was also the most prevalent (FO= 100% and PD=3560 nematodes /250g soil), flowed by Helicotylenchus and Pratylenchus (FO = 32-36 %  and PD= 45 -160 nematodes /250g soil) (Table 1).

Seven nematode genera were detected in the citrus soil samples collected from Kafr El-Sheikh governorate (Motobus location) Tylenchorhynchus was the most common (FO= 100% and PD= 7121 nematodes /250g soil). On the other hand, Helicotylenchus, Tylenchorhynchus, Pratylenchus, and Aphelenchoides were the least prevalent (FO= 1.3-8% and PD= 115-204 nematodes /250g soil) (Table 1).

Six nematode genera were recovered from the citrus soil samples collected from Kafr El-Sheikh governorate (Fuoa location ) T. semipenetrans was the most common (FO= 100% and PD= 4021 nematodes /250g soil), followed by Tylenchorhynchus and Trichodorus showed (FO= 14% and PD = 15-92 nematodes /250g soil).  However, Pratylenchus, Helicotylenchus, and Aphelenchoides were less common with 2-6% FO and PD of 13-34 nematodes /250g soil. (Table 1).

The survey studies evident that Nine PPN genera were found in association with soil and roots of the surveyed citrus trees, the nematode  genera T. semipenetrans was the most prevalent with (FO= 100%).

Table (1). Frequency of occurrence (FO) and population density (PD) of plant-parasitic nematode genera associated withcitrus  trees in Alexandria, El-Behera and Kafr El-Sheikh governorates

a =Number of collected samples.

FO = (Number of positive samples/total no. of collected samples) × 100.

PD = Mean number of nematodes/250g soil in the positive samples.