Abstract
Globodera pallida, the pale cyst nematode is a devastating pest of potato regulated internationally. In Idaho, G. pallida is a quarantine pest regulated by USDA-APHIS. Globodera pallida has potential to cause over 80% yield loss in severe infestations. Potato is one of Idaho’s most valuable crops. Because G. pallida could seriously threaten the Idaho potato industry, an eradication program has been developed for infested fields in Idaho. This dissertation presents research in which trap crops and crop rotations were evaluated as control strategies to support the on-going eradication efforts for G. pallida in Idaho. Additionally, the effect of varying initial infestation levels of G. pallida on resistant and susceptible potato was also investigated.Globodera pallida is a highly specialized plant-parasitic nematode that requires a specific hatching factor to hatch. Otherwise, G. pallida eggs within cysts can remain dormant but viable for decades in the absence of a host. The hatching factor is typically produced by species within the family Solanaceae. A trap crop for G. pallida in Idaho causes hatch but does not allow the nematode to complete its lifecycle. In this way, trap crops can reduce G. pallida population densities. Several solanaceous species were evaluated for their hatching stimulatory effect on and host status of G. pallida. All species under investigation were nonhosts. Solanum quitoense and Solanum retroflexum were found to cause G. pallida hatch comparable to that of potato.
Although quinoa (Chenopodium quinoa) is not a member of family Solanaceae, there was evidence that it was a nonhost that caused hatch of G. pallida. Quinoa production is becoming more popular in southeastern Idaho where infested acreage is located. Several Pacific Northwest-adapted quinoa varieties were evaluated for their G. pallida host status and hatching effect. All quinoa varieties were found to be nonhosts for G. pallida that also induced significant hatch of the nematode. The quinoa varieties tested induced 10 to 30% hatch of G. pallida eggs.
After confirming that the Pacific Northwest-adapted quinoa varieties were nonhosts that caused significant hatch of G. pallida, quinoa was evaluated as a trap crop under Idaho field conditions. The effect of quinoa was compared to that of Solanum sisymbriifolium, an effective trap crop for G. pallida. Solanum sisymbriifolium has been shown to reduce G. pallida populations 90% or more when in rotation with a susceptible potato. However, S. sisymbriifolium is not widely adopted by growers as a trap crop due to limited access to clean seed and lack of a marketable crop. In greenhouse and field trials, S. sisymbriifolium in rotation with a susceptible potato reduced G. pallida 82 to 99% while quinoa reduced G. pallida 37 to 45%. The rotation of S. sisymbriifolium, quinoa, or nonhost barley to a highly resistant potato was also examined. A commercially viable G. pallida resistant potato for Idaho is not yet available, so the European resistant variety ‘Innovator’ was used a model of how a resistant potato would impact G. pallida populations. Rotation to a resistant variety after the trap crop S. sisymbriifolium brought G. pallida populations to zero.
The effect of a resistant potato and trap crop S. sisymbriifolium was further investigated in three-year crop rotations. The hatching effect of a resistant potato and S. sisymbriifolium were compared and found to be similar to each other and to that of susceptible potato. The resistant variety when in rotation with S. sisymbriifolium reduced G. pallida populations similarly. Two years of S. sisymbriifolium reduced G. pallida to below a detectable level. These studies also provided further evidence that in addition to causing hatch of G. pallida, S. sisymbriifolium reduces viability of remaining encysted eggs more than other treatments, which may indicate a toxic effect that leads to reduction in egg viability.
This work also sought to determine the impact of varying initial G. pallida population density on yield and G. pallida reproduction on susceptible and resistant potatoes. The effect of the initial G. pallida population densities of 0, 10, 20, 40, and 80 eggs/g soil was investigated on the susceptible potato varieties ‘Désirée’ and ‘Russet Burbank’, and the resistant potato variety ‘Innovator’. Due to the quarantine status of G. pallida in Idaho, these experiments could not be conducted in the field. Instead, this study used greenhouse data integrated into a SUBSTOR-DSSAT model to simulate the effect of G. pallida on potato yield in Idaho field conditions. The SUBSTOR-DSSAT models for ‘Innovator’ and ‘Désirée’ predicted 50% yield loss at an initial population of 80 eggs/g soil. ‘Russet Burbank’, highly susceptible to G. pallida, allowed for greater G. pallida reproduction than ‘Désirée’. However, greenhouse studies suggested that ‘Russet Burbank’ is tolerant of G. pallida as yields were not significantly reduced at any initial population density. Due to numerical reductions in ‘Russet Burbank’ yield, SUBSTOR-DSSAT models predicted about 10 to 20% yield loss at high infestation levels. However, at high initial population densities, ‘Russet Burbank’ experienced reduced tuber size and increased numbers of tubers which reduce yield quality. The data also demonstrated that at high population densities, G. pallida reproduction on susceptible potato was limited by competition. These results help illustrate the effects of G. pallida on potato in Idaho and demonstrate G. pallida population dynamics on susceptible and resistant potato.
Ultimately, the knowledge gained from this research can be applied to G. pallida control strategies. These findings also advance understanding of how a highly resistant potato variety can impact G. pallida population dynamics and how a resistant and susceptible potato can have different degrees of tolerance to G. pallida. The simulations of potato yield for these potato varieties further illustrate the potential devastating impact of G. pallida on potato yield.