An Algorithm for Optimizing Rotational Control of Globodera rostochiensis on Potato Crops in Bolivia.

The outline area of new cysts of Globodera rostochiensis was measured by image analysis. A linear regression of this value against egg content provided a basis for adjusting the egg number for cyst size. This adjusted egg content provides an estimate of the relative fullness of a cyst with eggs. This value showed an exponential decline in eggs over 3.5 years since the last potato crop. It corresponds to an average loss in the dormant egg population of 32.8 +/- 5.6%/year for 26 fields at Toralapa, Bolivia. This value compared well with a mean decline of 40 +/- 4%/year for 42 fields after measuring viable eggs/100 g soil on two occasions one year apart. The new approach allows declines to be estimated at one time point. The decline in lipid content of the dormant, unhatched second-stage juveniles (J2) was 17 +/- 6% per annum as measured by image analysis after Oil red O staining. This may be sufficient to compromise infectivity after 3 to 4 years of dormancy. A standard model was modified to consider the effect of both lipid depletion during dormancy and choice of susceptible potato on the population dynamics of G. rostochiensis under rotational control. It is concluded that a few cultivars may impose lower populations on G. rostochiensis in 3 to 4-year rotations than the majority used in Bolivia.

The Influence of Potato Cultivar on Lipid Content and Fecundity of Bolivian and British Populations of Globodera rostochiensis.

The influence of host cultivar on the lipid levels provided by a female to her progeny was investigated with Oil Red O stain and a quantitative image analyzer. A population of Globodera rostochiensis was multiplied at Toralapa Field Station in Bolivia on 25 different potato cultivars grown in that country. The mean neutral lipid content of newly formed second-stage juveniles varied significantly with cultivar over a 200% range. The corresponding range was only 18% and 28% for the same Bolivian and a UK population of G. rostochiensis, respectively, when both completed reproduction concurrently on 10 pot-grown European cultivars in the United Kingdom. Egg numbers per female varied with host for Bolivian cultivars that lack known partial resistance to Globodera spp. There was a 15-fold range between the most and least fecund nematode-host combinations (Kosi and Gendarme). The Bolivian G. rostochiensis population showed only a 2-fold range in mean eggs per cyst when grown on European cultivars in the UK. The fatty acid profiles of lipids from Bolivian G. rostochiensis cysts reared on Bolivian potato cultivars were dominated by C(20) (37-64%) and C(18) (28-46%) fatty acids and ranged from C(14) to C(22). The three major fatty acids detected were C(20:4:), C(20:1), and C(18:1). Few differences between cultivars were observed. For a UK population of G. rostochiensis reared on ssp. tuberosum, higher relative percentages of C(18) and monounsaturated fatty acids and lower relative percentages of C(20) and polyunsaturated fatty acids were found.

Changes in the lipid content and fatty acid composition of 2nd-stage juveniles of Globodera rostochiensis after rehydration, exposure to the hatching stimulus and hatch.

The total lipid content of the dry weight of whole cysts and 2nd-stage juveniles (J2) of Globodera rostochiensis was 17.1% in dry cysts, 20.9% in cysts soaked in distilled water (DW), 20.3% in cysts that had been in potato root diffusate (PRD) for 7 days, 7.3% in cysts that had been in PRD for 28 days and 29.2% for hatched J2. The fatty acid composition of the total lipid did not differ between dry cysts, cysts in DW and cysts in PRD for 7 days. However, major differences in the fatty acid composition of all lipid classes were found between rehydrated cysts in PRD and freshly hatched J2. After hatching, the degree of saturation and the percentage of monounsaturated fatty acids decreased and the percentage of polyunsaturated fatty acids increased considerably, especially in the free fatty acid fraction, where C20:1 showed an 8-fold decrease and C20:4 a 33-fold increase. There was a difference in the fatty acid composition of cysts in PRD for 7 days and cysts in PRD for 28 days (after most of the J2 had hatched); with increased time in PRD the percentage of polyunsaturated fatty acids increased and the percentage of monounsaturated fatty acids decreased in all lipid classes. Differences in the fatty acid profiles between cysts in PRD for 28 days and hatched J2 were found mainly in the free fatty acid and the non-acidic phospholipid fractions. The free fatty acid fraction of the cysts was dominated by monounsaturated fatty acids (52%), whereas the same fraction of hatched J2 contained large amounts of polyunsaturated fatty acids (60%). These polyunsaturated fatty acids, especially C20:4, might play an important part in nematode-plant interactions during infection.

Seasonal Changes in the Dorsal Pharyngeal Gland Nucleolus of Unhatched Second-Stage Juveniles of Globodera spp. in Bolivia.

Changes in the diameter of the nucleolus of the dorsal pharyngeal gland (DPGN) in unhatched second-stage juveniles (J2) of potato cyst nematodes, Globodera spp., were monitored for cysts recovered during two field experiments in the Bolivian Central Andes. In the first experiment, cysts were extracted from soil left fallow or supporting crops of potato, barley, lupin, or quinoa. The highest mean DPGN diameter for unhatched J2 occurred shortly after planting in January. The values were similar for individuals recovered from cysts associated with all cultivations. For cysts from potato plots, the lowest mean DPGN diameter of 2.26 +/- 0.05 mum occurred in March, but the value increased again by May to 2.53 +/- 0.05 mum. Similar seasonal changes were found for J2 under both nonhost crops and fallow with the smallest diameters recorded in May of 2.48 +/- 0.02 mum and 2.34 +/- 0.05 mum, respectively. Two possible factors might cause this significant seasonal change. First, some J2 may hatch early in the growing season, even in the absence of the host. This would enhance the proportion of dormant, unhatched J2 remaining in the cyst samples. Secondly, a seasonal change in the DPGN diameter may occur for most individuals with a transient fall value between January and March/May. A model defined by this study provides a good description of the observed effect, providing both factors are assumed to occur. The second experiment studied if changes in size of DPGN in response to a hatching stimulus are influenced by the cyst population age. The DPGN in unhatched J2 was measured for cysts recovered from soils that had supported potatoes that growing season or 2 or 4 years earlier. The unhatched J2 from the freshly cropped potato site showed the largest mean DPGN diameter of 3.66 +/- 0.05 mum after 7 days in potato root diffusate, whereas those from the 4-year sample had the smallest value of 3.20 +/- 0.05 mum. This significant difference may indicate a delayed response to the hatching stimulus with more prolonged J2 dormancy.