Negin Ebrahimi

Observations on the life cycle of potato cyst nematodes, Globodera rostochiensis and G. pallida, on early potato cultivars

Potato cyst nematodes (PCN) are the most economically important nematode pests of potato. Early harvesting is part of a preventive management approach for very early potato varieties. In Belgium, tubers are harvested before 20 June because it is assumed that no cyst formation occurs by that date. However, this assumption might not be valid any more because of climate change, the availability of new cultivars (with new traits), and the increasing prevalence of Globodera pallida. Therefore, pot, microplot and field trials were conducted to study the life cycle of PCN in early potato-growing conditions. The development of populations of G. pallida on three potato cultivars, Eersteling (susceptible to PCN), Premiere (resistant to G. rostochiensis) and Ambassador (partially resistant to G. pallida) and that of G. rostochiensis on cv. Eersteling was monitored in the growth chamber with simulation of field temperatures of the season. On cvs Eersteling and Premiere, second-stage juveniles (J2), males, females and cysts of G. pallida populations were found 28, 56, 63 and 77 days after infestation with cysts (DAI), respectively. The number of degree days (DD) for G. pallida to complete its life cycle using a base temperature of 4°C was calculated 450 DD4. On cv. Ambassador, females of G. pallida were never observed. J2, males, females and cysts of G. rostochiensis were detected 42, 70, 70 and 84 DAI, respectively. This species needed 398 DD6 at a base temperature of 6°C to complete its life cycle. Observations in two fields and in microplots under prevailing weather conditions in 2013 revealed that cysts of G. pallida and G. rostochiensis were formed on June 12, when the accumulated degree days were 463 DD4 and 401 DD6, respectively. Our observations show that both species of Globodera develop earlier than was assumed based on previous data. Therefore, harvesting based on the accumulated heat above the basal development temperature required by PCN species can replace the set harvest date.

Optimizing trehalose-based quantification of live eggs in potato cyst nematodes (Globodera rostochiensis and G. pallida)

Integrated management of potato cyst nematodes (PCN; Globodera rostochiensis and G. pallida) relies heavily on the determination of cyst population densities in soil as well as the viability of the eggs inside the cysts. This study aimed to optimize a quantitative method to determine the number of viable eggs of PCN based on trehalose present in live eggs. Trehalose was extracted from cysts and from a dilution series of eggs and quantified. More trehalose was detected when cysts were crushed than when left intact. Reaction volumes were adapted to the number of eggs because small reaction volumes hampered an accurate extraction of trehalose. A maximum of 10.5 eggs/μl of reaction volume should be used to obtain a significant linear relationship between detected trehalose content and egg numbers. The sensitivity of the trehalose-based method was evaluated by determining the lowest egg detection limit and was defined as five viable eggs. The reliability of this method was tested by comparing efficacy with that of two commonly used assays, visual assessment and hatching test. The trehalose-based method gave viability results similar to those of the visual assessment, which is time consuming, requires trained personnel, and can involve some subjectivity. The hatching test identified fewer viable eggs than the other two methods. In addition, the viability of dead eggs (heated and naturally dead) was tested. No false-positive results (dead eggs declared viable) were obtained with the trehalose-based method. The robustness of the test was demonstrated by measuring the viability of eggs of PCN in different experiments repeated in time. The viability assessment method based on trehalose proved to be an objective as well as sensitive, reliable, robust, fast, and cheap technique for assessing the number of viable eggs in PCN cysts.