Pathogenicity and Host–Parasite Relationships of Heterodera cruciferae in Cabbage

Sasanelli, N., Vovlas, N., Trisciuzzi, N., Cantalapiedra-Navarrete, C., Palomares-Rius, J. E., and Castillo, P. 2013. Pathogenicity and host–parasite relationships of Heterodera cruciferae in cabbage. Plant Dis. 97:333-338. Stunted cabbage (‘Lupini’) associated with severe soil infestations by a cyst-forming nematode were observed in large patches of open fields in Castellaneta, province of Taranto, southern Italy. Morphological traits based on mature cysts, males, and second-stage juveniles (J2s) and molecular analysis of ribosomal DNA (D2 to D3 expansion segments of 28S and internal transcribed spacer [ITS]1 region) were used to identify the species. ITS1 sequence information supported the identity of Heterodera cruciferae, also showing a high degree of similarity to other species of the Heterodera Goettingiana group, including H. goettingiana, H. carotae, and H. urticae. Nematodes successfully established permanent feeding sites in cabbage roots which caused cellular alterations in the root cortex, endodermis, pericycle, and vascular cylinder by inducing typical multinucleate syncytia. Syncytial cytoplasm was granular and dense, with variously sized vacuoles and hypertrophied nuclei with nucleoli. Cabbage plant growth was also reduced in pathogenicity tests. The relationship between the initial nematode population density in soil and shoot plant weight was well described by the Seinhorst’s equation. Tolerance limits with respect to shoot plant weight of cabbage to H. cruciferae was estimated as 1.50 units of eggs plus J2s/cm3 of soil. The minimum relative value (m) for plant height was 0.71 at an initial nematode population density of (Pi) ≥ 64 units of eggs plus J2s/cm3 of soil. The maximum nematode reproduction rate (Pf/Pi) was 4.6 times that of the initial population density of 8 units of eggs plus J2s/cm3 of soil. Cabbage (Brassica oleracea var. capitata) has long been cultivated during cool seasons in temperate climates as an economically important vegetable crop in several Mediterranean countries (8). In Italy, it is widely cultivated (about 18,000 ha), especially in the southern regions. In the Mediterranean basin under field conditions, cabbage is usually transplanted in late summer and harvested in early spring. The diseases it can be affected by are those caused by plant-parasitic nematodes, which can cause severe damage (1,20–23,28), particularly the cyst-forming species Heterodera cruciferae and H. schachtii (2,4,17–19). Damage to vegetable crops is influenced by specific Heterodera spp. and the initial nematode population density in soil at the time of sowing (24). Because Heterodera spp. are often host specific, precise identification and estimation of population density in soil are crucial for designing effective control measures in the context of sustainable and integrated pest management, and considering their long persistent dormant stage (eggs protected within cysts). It is well established that the extent of crop growth suppression is influenced by the nematode population density at sowing and that a minimum population density (T) is required before measurable yield loss occurs (tolerance limit) (25,26). Although damage caused by the cabbage cyst nematode H. cruciferae to cabbage is known in several growing areas such as Europe, California, Russia, South Australia, and Turkey (4,6,9,11,19,29), some controversy exists regarding the pathogenicity, threshold levels, and host–parasite relationships on cabbage. Some authors have not considered H. cruciferae to be a major pest of cabbage (29) but others have reported considerable crop losses (18,33). The sugar beet cyst nematode H. schachtii has also been reported as damaging cabbage and occurring together in the same fields with H. cruciferae (28). Although H. cruciferae and H. schachtii have many common hosts, their host range is somewhat different, as is their pathogenicity on cabbage (18). Hence, proper identification is necessary for selecting control measures. Under glasshouse conditions, McCann (18) reported that H. schachtii and H. cruciferae reduced shoot and root growth of cabbage, with the former causing higher levels of damage. However, no data are available on the minimum population density of the cabbage cyst nematode determining the threshold for measurable yield loss (tolerance limit T) (25). During early autumn 2010, severe feeder root infections of ‘Lupini’ cabbage and soil heavily infested (64 units of eggs and second-stage juveniles [J2s]/cm of soil) by Heterodera spp. were found in fields at Castellaneta (Taranto Province) in southern Italy. Areas of affected plants occurred in patches within the field. Diseased plants showed damage that occurred in patches within the field, with symptoms including delayed formation of heads, severe stunting, and heavily affected root systems. The abundance of cystnematode-affected roots suggested a highly specialized nematode– plant interaction. Therefore, the objectives of this study were to determine (i) accurate identification of the nematode species, (ii) the histopathology of nematode-feeding sites, and (iii) the relationship between the initial soil nematode population density and growth of cabbage under greenhouse conditions. Materials and Methods Nematode identification. The cyst nematode isolate infecting cabbage roots was identified by morphology, as well as by molecular analysis of ribosomal DNA. Four samples of infected cabbage roots with soil of the associated rhizosphere as well as bulk soil were taken arbitrarily with a shovel from the upper 30 cm of soil from the field site in October 2010. Cysts, males, and J2s were extracted from soil (5), and adult white females and cysts were also recovered from root tissues, mounted in glycerin, and examined by Corresponding author: P. Castillo, E-mail: [email protected] *The e-Xtra logo stands for “electronic extra” and indicates that Figures 1 and 3 appear in color in the online edition. Accepted for publication 4 September 2012. http://dx.doi.org/10.1094 / PDIS-07-12-0699-RE © 2013 The American Phytopathological Society e-Xtra*