Ecological Management of Potato Cropping Systems (ANE93.018)

Objective 1. The soil amendment treatments increased soil organic matter, aggregation, and fertility after a single season. After three years of application, the soil amendment treatment had significantly increased soil organic matter content (3.74 vs. 4.94%), cation exchange capacity (5.7 vs. 7.2 meq 100/ gm ), and the soil test levels of phosphorus (37.4 vs. 42.5 kg / ha), potassium (298 vs. 551 kg / ha), calcium (1366 vs. 1906 kg / ha), magnesium (429 vs. 524 kg / ha), and zinc (0.91 vs. 1.28 kg / ha). Nitrate-N concentrations in the surface soil during 1995 were increased due to amendment application. Soil amendments significantly decreased soil bulk density (1.24 vs. 1.14 g cm-3) and increased water stable aggregate content of the soil (37.9 vs. 43.4%). Average size of the soil aggregates was increased. Gravimetric moisture sampling revealed that the amendment program increased soil moisture levels throughout much of the growing season by about 10% and that plots receiving both soil amendment treatment and supplemental irrigation had particularly high soil moisture content. We were not able to detect any effects of the soil amendment treatments on water infiltration rates (unamended range 4.6 to 48.0 cm / h; amended range 3.6 to 71.7 cm / h) or laboratory-measured, soil-moisture-retention characteristics. The green manure crop rotation significantly increased nitrate-N levels within the soil, but had no other significant effects on soil properties. While the observed soil fertility increases due to amendment applications are generally desirable for crop production, environmental degradation could occur if these nutrients move off site due to runoff or leaching.

Runoff collectors were established on selected experimental plots during 1994 and 1995. Samples collected during 1995 indicated that irrigation treatments significantly increased runoff volume by 85%, but had no effect on sediment movement. Amendment application did not significantly affect runoff volume or sediment movement. Based on the changes in soil physical properties described above, the RUSLE was used to evaluate the potential for the soil amendments to decrease soil erosion. The seasonally adjusted soil erodability factor (Kf) was projected to drop from 0.243 to between 0.198 and 0.216, depending on the assumptions used. Averaged over a two-year crop rotation, the estimated soil erosion on a Caribou loam soil in Maine (91 m slope @5%) would drop from 5.10 to between 4.14 and 4.55 Mg / ha / year.

June through August rainfall for 1993-95 totaled 27, 23, and 16 cm, respectively. Irrigation increased yields by 3.14, 7.84, and 12.1 t / ha over the three respective years. Amendments significantly increased total yields in all three years of the study (8.62, 8.18, and 3.36 t / ha, respectively). US#1 yields were also increased (3.25, 7.84, and 2.69 t / ha, respectively). The rotation crop treatments did not significantly affect yields or US#1 yields. The higher N rate increased yields during 1993 (1.57 t / ha), but decreased yields during 1994 (2.46 t / ha). Contrary to expectations, total and US#1 yields at the lower N rate (134 kg N / ha) were not higher in the green manure rotation than the oat rotation. The 1993 results indicate that the green manure rotation crop as managed in this study was not able to greatly enhance productivity via N delivery to the subsequent crop. During 1994, 134 kg N / ha provided sufficient N to maximize yields even when oats were the previous crop. In parallel studies we have shown that the green manure crop can decrease the N fertilizer requirement by 67 to 134 kg / ha (33 to 66%). There were no significant interactions among irrigation, rotation crop, or soil amendment treatments.

During 1993, total yields in the non-irrigated, amended treatments were higher than those in the best irrigated, unamended treatment; however, the amended treatments were not able to maintain this advantage during the drier 1994 and 1995 growing seasons. Yields in the non-irrigated, amended treatments were 92 and 82% those of the irrigated, unamended treatments. In comparison, yields in the non-irrigated, unamended treatments were only 72 and 69% those of the unamended, irrigation treatments. From this, the yield enhancing effects of the soil amendment treatment make it an attractive alternative to supplemental irrigation; however, the soil amendment program was not able to entirely compensate for the dry conditions experienced during 1994 and 1995. It is noteworthy that yields were consistently higher for the irrigated, amended treatments than the irrigated, unamended by an average of 7.06 t / ha or 19%. Only the irrigated and amended treatments consistently reached or exceeded our target yield potential of 41 t / ha, suggesting that the soil structural and nutrient supply improvements provided by the amendments can be beneficial even in irrigated systems. It is not possible to determine how much of the response was due to nutrient supply and how much was due to changes in soil physical properties.

Irrigation increased potato top growth from mid-season onward during 1994 and 1995. The soil amendment treatments enhanced early growth of the potato crop and increased the size of the plant canopy. As was expected, irrigation resulted in leaf water potential (LWP) measurements that were closer to zero than those obtained for non-irrigated plots. Soil amendment treatments did not significantly affect LWP at any sample date; however, they did result in lower stomatal resistance and increased transpiration rates on several sample dates. These results indicate that the soil amendment treatments did improve crop water status; however, the extent to which they did so is difficult to establish because of the temporal changes in water supply over the season and the expectation that the larger crop canopy in the amended treatments would deplete soil water more rapidly than the smaller, unamended crop canopy.

Irrigation increased tuber size during two of the three study years, while the soil amendment treatment increased tuber size only during 1994. There were no treatment effects on incidence of scabby, sunburned, misshapen and growth-cracked tubers. Tuber decay incidence was very high during 1993, but relatively little tuber decay occurred during 1994 and 1995. Supplemental irrigation dramatically increased the incidence of tuber rot during 1993 (17.2 vs. 41.3%) and also increased rot incidence during 1994 (0.1 vs. 1.1%). Soil amendment also tended to increase the incidence of tuber rot (1993: 25.7 vs. 29.7%; 1994: 0.4 vs. 0.7%; 1995: 0.3 vs. 1.0%), possibly because it results in increased soil moisture levels throughout much of the growing season. Soil amendment did not dramatically affect the incidence of hollow heart in these experiments, but it decreased specific gravity in two of three growing seasons (1993: 1.078 vs. 1.075; 1995: 1.076 vs. 1.071). Crop rotation did not generally affect tuber quality in this study.

Supplemental irrigation is a proven tool for enhancing potato yields in Maine, yet it is an expensive technology that is not readily accessible to all growers and cannot be readily used on all farms. Soil amendment application is a promising tool for improving soil quality and enhancing potato yields in Maine; however, application can initially be costly and refinement of the amendment programs will be needed to accurately assess the economic impact of this approach. The current soil amendment system was designed to rapidly build soil organic matter and was not designed to optimize the rate of amendment application. It is likely that amendment rates and frequencies of application were much higher than were necessary for obtaining the yield increases observed in this work. In this experiment, we did not lower fertilizer rates to compensate for the nutrients present in the amendments. Other experiments have clearly indicated that fertilizer cost savings can be achieved when these amendments are utilized. For example, a similar soil amendment system in a related potato cropping systems study significantly increased potato yields during 1994 (4.0 t / ha, 12%) and 1995 (5.5 t / ha, 30%), while two-year average N fertilization rates were decreased by 65%, P2O5 by 50% and K2O by 75%. In addition to the nutrient responses that could not be addressed within this study, it is extremely likely that the positive effects of the amendments on soil properties and crop yields will last for several seasons after amendment applications have ceased. The potential of soil amendments should be studied over a much longer time period before any conclusions are drawn as to their true feasibility for Maine producers. Potential impacts on plant diseases and tuber decay should also receive more attention.

Objective 2

Small Plot Tests: Evaluation of Augmentative Releases of P. bioculatus in Combination with Bt and B. bassiana.

The 1993 CPB data from outside the cages indicate that there were significantly fewer CPB adults and fourth instar CPB in the Bt treatments than the remaining treatments. Fourth instar densities were significantly lower in all treatments compared to the check with densities lowest in the Bt plots (0.05 / plant), and lower densities in the B. bassiana plots (1.26 / plant) than in the P. bioculatus plots (1.38 / plant). There were no treatment effects on egg mass or first instar larval densities, and no interactions between any of the natural enemy treatments. Densities of CPB second and third instar larvae were reduced by the Bt treatments, but no other significant differences were observed between B. bassiana or P. bioculatus treatments and the control.

Analysis of the 1993 cage data revealed no impact of C. maculata releases on total emergence of adult CPB; however, Bt, B. bassiana, and P. bioculatus significantly reduced the number of emerging adults. The lowest mean emergence was observed in plots with all three mortality agents, however, P. bioculatus alone resulted in significantly higher adult emergence than the remaining non control treatments. There were no differences in the densities of emerging adults between Bt treatments and B. bassiana treatments. These results indicate that considerable additional mortality occurred during the pupal stage in the soil in the B. bassiana treatments.

The 1994 results of the small plot study at both sites were similar to the 1993 results; however, several treatment effects were detected earlier in the population than during the previous year. Egg mass densities were significantly reduced in the Bt plots at the Presque Isle site. P. bioculatus reduced small larva densities at the Old Town site (1.67 + 0.48 first instars in the predator versus 4.26 + 1.11 in the non predator plots). Predators significantly reduced densities of second, third, and fourth instars in Presque Isle, whereas in 1993, P. bioculatus effects were not detected until the fourth instar stage. Bt effects were also detected in second (p=0.03) and third (p=0.01) instar densities in Old Town, and all instars at the Presque Isle site. In all cases, densities of CPB larvae were lower in all Bt plots versus non Bt plots. B. bassiana treatments significantly reduced fourth instar densities in Presque Isle, but not at the Old Town site. Interactions between treatments were detected for only egg mass densities at the Presque Isle site. Bt treatments without predators appeared to have little effect on egg mass densities at this site, however, Bt in combination with P. bioculatus resulted in fewer egg masses than P. bioculatus applied by itself.

Defoliation ratings ranged from 0.3% for plots with the combination of all three biological control agents at both sites to 54.3% and 27.6% in the controls in Old Town and Presque Isle, respectively . Defoliation ratings in all biological treatments were significantly less than the control, but not different from each other in Old Town. However, the B. bassiana alone treatment did result in more defoliation than the other treatments in Presque Isle. In Old Town, the B. bassiana plots did not have significant reductions in CPB larval densities, but resulted in only 6.2% defoliation. Large larvae were observed on the foliage, but most were infected and caused little feeding damage.

Emergence of summer adults at both sites was significantly reduced by all biological agents, compared to the control. There were no significant differences between the treatments receiving some form of CPB control, but the lowest mean emergence at the Presque Isle site was observed with the combination of three agents. In addition, 42% of the adults emerging in B. bassiana treatments at the Old Town site died within 10 days of emergence, whereas, only 15% of those in non B. bassiana plots died over the same period. There was no difference in post emergent mortality of adults at the Presque Isle site; however, there was a significant B. bassiana effect on the number of collected larvae that died post spray at both sites. In Old Town, 61% of those collected from B. bassiana plots died compared with 42% from Bt, 44% from the B. bassiana + Bt, and 21% of the control. In Presque Isle, 90% of those collected from B. bassiana plots died compared with 53% from Bt, 95% from the B. bassiana + Bt, and 19% of the control.

Comparisons of half and full rates of Bt with and without P. bioculatus, did not result in any significant differences in CPB densities between treatments. However, mean densities of large larvae were consistently lower in the Bt + P. bioculatus plots compared with the other treatments, with no difference between the full Bt rate with P. bioculatus and the half Bt rate with P. bioculatus. Emergence of summer adults was significantly greater in the control plots than all other treatments, but half rate Bt did not differ from the full rate of Bt. Again, percent defoliation was low in all of the treatments and was significantly less than that observed in the controls.

Comparisons of consecutive applications of B. bassiana compared with applications applied at weekly intervals showed no significant differences between treatments in the densities of immature life stages of the CPB. The trends in the means showed lower emergence and defoliation in the B. bassiana treatments than the controls, but no difference between application strategies. Emergence averaged 8.0 + 3.5 for controls versus 1.3 + 0.9 for weekly B. bassiana applications, and 2.0 + 1.5 for consecutive B. bassiana applications. Defoliation averaged 54.3 + 25.7% for controls versus 6.2 + 2.2% for weekly B. bassiana applications, and 4.3 + 0.8% for consecutive B. bassiana applications.)

Systems Study. A System Level Comparison of Pest Management Programs

In 1993, CPB densities exceeded the threshold an average of three times during the growing season in CONV, once in RI, and once in BIO. As a result, the CONV plots received three treatments with Asana7, the RI plots received one application of Asana7, and the BIO plots received one application of Bt and B. bassiana in combination. The incidence of CPB adults was significantly lower in BIO than RI, and the seasonal densities of eggs, small and large larvae were all lower in BIO than CONV and RI. As might be expected given the higher thresholds, there were more small and large larvae in RI than in CONV. All of these strategies successfully reduced CPB populations below economic thresholds. There were no significant differences in yields between pest management treatments.

In 1994, CPB densities exceeded thresholds an average of three times in CONV, twice in RI, and 2.75 times in BIO. The CONV and RI plots were again treated with Asana7. The BIO plots received two applications of Bt and B. bassiana for larvae, and three out of four blocks were treated with rotenone for summer adults. In 1994, densities of all CPB lifestages were significantly lower in BIO compared with both RI and CONV. In this on going study, CPB populations have been declining in the BIO relative to the other treatments since 1992. The proportion of adults in the plots prior to the applications of any foliar treatments or predator releases has been lower in the BIO plots compared with the other treatments.

In 1996, attempts were made to implement the biological management program for the Colorado potato beetle on two commercial farms: Wood Prairie Farm in Bridgewater, ME, and Gorenson Farm in Dresden Mills, ME.

At Wood Prairie Farm, a field of potatoes was divided into 6 sections for comparing different combinations of foliar formulations of Bt (Novodor 3%7 provided by Abbott Laboratories, Chicago, IL), and B. bassiana (Mycotrol7, provided by Mycotech Bioproducts, Butte, MT), with and without releases of the predatory stinkbugs, P. bioculatus. The planned foliar treatments included: full rate of Bt alone, full rate of Bt with B. bassiana, and 2 rate Bt with B. bassiana. Three releases of stinkbug predators were made at a rate of one nymph per plant on June 26, July 3, and July 10. CPB densities in all sections (with and without predators) never reached levels requiring foliar applications. Over the season there were no significant differences in CPB densities between sections with and without releases of the predators, although there was a trend for both fewer small and large larvae in areas where the predators were released. CPB populations will be monitored in 1997 to see if there is any evidence of a carry-over effect from the B. bassiana applications.

At Gorenson Farm, two fields of potatoes were divided into two sections. One section received foliar applications of Bt and the other section received foliar applications of Bt with B. bassiana. No predators were available for release at this site. Four foliar applications were made on all sections between June 25 and July 22. In field #1, relative densities of CPB larvae were higher in the Bt plus B. bassiana treatment section than the Bt alone section. However, in field #2, the opposite was true. In field #1, the section treated with Bt alone was planted and harvested early (starting in mid-July), and there was considerable migration of larvae from this area to the adjacent section which was treated with Bt plus B. bassiana.