Assessment and Demonstration of the Sustainability of Long vs. Short Potato Rotations

Nematode

Several different nematode species were identified in 2007 (Table 1) and 2008 (Table 2). Cyst Heterodera, Columbia Root Knot, and Stunt nematode species were identified in some fields in both years and Northern Root Knot, Pin, and Golden nematode species were identified in some fields in 2007 only.

Analyzing the data by year, species, and rotation length showed that there was no significant interaction for year in any comparison and, therefore, the data was combined across years for the final analysis. There was a significant species by rotation length interaction. Columbia root knot nematode was the only species having a significant increase in population in both the number of fields infected and the population in these infected fields (Figure 1).

It is important to note that only 6 out of the 54 short-rotation fields were infected with Columbia Root Knot nematode, but the populations were very high in comparison to the long-rotation fields where only 2 of the 54 fields were infected.

These findings are particularly interesting, considering the fact that a much higher percentage of the short-rotation fields (50 out of 54) were fumigated during the year prior to sampling compared to long rotation fields (19 out of 54), as well as having more fumigation events in previous years than the long-rotation fields. Although it is surprising that most of the nematode populations were not affected by rotation length, the magnitude of the differences for Columbia Root Knot nematode proved to be an exception despite the
tremendous increase in fumigation incidence.

Increased nematode occurrence affects potato production in numerous ways. The increased populations not only potentially reduce yield and quality, but also likely increases grower expenses for nematicides and fumigants. Chemical control of some nematode species is expensive, often exceeding $300 per acre. It has been determined by survey that growers with fields in an every other year potato cycle nearly always need to fumigate, but growers with fields where potatoes are grown every four or more years oftentimes do not need to fumigate.

Wireworm

There were no significant differences in wireworm infection in the field stands or the harvested tubers from the greenhouse studies (data not shown) between long and short rotation soils evaluated in this study, however, it is important to note that, in both 2007 and 2008, there was very little to no wireworm damage observed by potato growers in the surrounding areas. This was confirmed by Dr. Juan Alvarez who stated that “wireworm pressure was very low across the region in 2007 and 2008”. Therefore, it is not possible to make conclusions about the impact of rotation length on wireworm infection. More study is needed under conditions of higher wireworm infection.

Pathology

In field visual analysis showed that the incidence and severity of Verticillium dahlia was significantly greater for short vs. long rotation fields. Nearly all short rotation fields (53 of 54) had incidence of this pathogen within the sampling area; whereas fewer of the long rotation fields (37 of 54) had incidence. On a visual rating scale of 0 (no damage) to 5 (complete necrosis), the short rotation fields had an average of 3.2 and the long had an average of 2.3.

Incidence and severity of several of the other evaluated pathogens (Rhizoctonia, white mold, pink rot, and Pythium leak) were all numerically greater for short vs. long rotation fields, but the differences were not significant. Incidence and severity of silver scurf and powdery scab were minimal and not significant.

These results were confirmed in the greenhouse study where there was minimal infection of most soilborne pathogens, except for Verticillium dahlia, which was present at high levels and dominated the rate of senescence and discussed below.

Weed Identification, Germination and Count/Herbicide Resistance Studies

After 2 weeks, only a very few scattered yellow foxtail (Seteria viridis) and pigweed sp (Amaranthus sp) plants germinated throughout the 18 pots, which was not a critical mass, so the potting was repeated with more soil from the sample bags. After a 3 week period under the same growing conditions as described above, only a few scattered weeds germinated, which was again, not enough to test. As minimal weeds germinated in these repeated attempts, the preemergence herbicide trial was not conducted.

There was a question as to whether or not herbicides the growers had applied to the test fields might still be present in the soil samples and therefore affecting weed seed germination.

• A bioassay was conducted by planting 4 tame oat seeds into the already potted soil samples.
• By 2 weeks after planting, 95% or greater oats had emerged with no sign of herbicide symptoms.

Although the soil-sampling and weed-seed planting projects reportedly kept the soil samples cool, dry, and dark, the samples may have been exposed to a short period(s) of high temperatures, which in turn, may have killed the weed seed in the samples. Since sample handling was well monitored, the possibility of this event occurring is quite remote. If the soil was somewhat moist inside the sample bags, weed seeds possibly germinated and died before the samples were processed at the Aberdeen R&E greenhouse.

However, when the soil was mixed for greenhouse planting, no weed seedling “skeletons” were noticed. It could be that weed seeds did germinate and die in the sample bags but became too desiccated by greenhouse planting time to be recognizable. In our experience, germinated weed seeds in soil samples are recognizable for a long enough period of time to have been seen during this trial. Even if this did occur, one would expect enough weed seeds in the samples to have emerged once the soil was potted.

Otherwise, no reasonable explanation for the unsuccessful weed seed germination seems plausible to this project and further study in needed in order to draw conclusions regarding the impact of short vs. long rotation length on weeds in potato.

Soils

There were no impacts of rotation length on soil carbon, mineral nutrients, bulk density, aggregate stability, or water infiltration (data not shown).

Greenhouse Container Study

The plants grown in the short-rotation soils had lower visual ratings (evaluations performed blindly – without knowledge of which plants were grown in short vs. long rotation soils) and began senescence an average of 15 days earlier than the plants grown in long-rotation soils (Figure 2). Total senescence was achieved 5 days later for potatoes grown with the long-rotation soils. The potatoes grown in long-rotation soils also had numerically higher tuber size, yields, and percentage of U.S. No. 1 potatoes (Figure 3). It
should be noted that tuber size was small due to constraints of growing tubers in buckets in a greenhouse environment.

In comparison to plants grown in the long-rotation soil, plants grown in short-rotation soils:

• began senescence an average of 15 days earlier
• produced 19.6% less tuber yield
• yielded no effect on root growth (Fig. 3)
• generated no differences for tuber numbers (data not shown)

As compared to long rotations, short-rotation practices lead to earlier senescence and lower tuber yields.

Based on previous research that shows an average yield loss of 7 cwt per acre per day, the five-day difference in total senescence could lead to a potential yield loss of 35 cwt per acre for the short-rotation fields. Since senescence began 15 days earlier, this could result in even greater yield losses--up to 105 cwt per acre based on the 7 cwt per acre loss for 15 days. Although larger in magnitude, this finding is similar to the yield loss of approximately 60 cwt per acre that was shown in previous field studies on short- vs.
long-rotation fields (Dr. Jeff Stark, University of Idaho at Idaho Falls, personal communication). The difference in magnitude could be due to the longer time frame in which the soils used in our study were subjected to short- or long-rotation practices.
The likely reason for this response is due to increased pest/pathogen pressure (Stark and Love, 2003). In our study, the early senescence was caused by verticillium wilt (early die complex); although this conclusion was based solely on visual observation (lab results are pending). Nematode damage was also likely (Tables 1 and 2), although direct damage to tubers was not observed. Damage instead could have been to roots. 

These results confirm the hypothesis that potato plants grown in short-rotation soils would likely require more pesticides relative to potato plants grown in long-rotation soils. With this confirmation, researchers can now proceed to the next steps: quantification of single factors (nematodes, pathogens, insects, etc.) to determine exactly why long crop rotations are more productive and to study their economic impacts.

Fry Quality Analysis

Post-harvest fry color rating (bud end, stem end and USDA rating), mottling (severity and percent incidence), percent sugar ends and sugar content are summarized in Table 3. In general, the long-rotation fields had numerically higher values/scores on the fry quality parameters compared with the short-rotation fields. Overall, values for both rotation lengths were within the standard range.

Post-storage fry color rating, mottling, percent sugar ends and sugar content are summarized in Table 4.

Once again, the long-rotation fields, in general, had higher numerical higher value/score on the fry quality parameters compared with the short-rotation fields. The percent incidence of mottling was significantly higher in both short and long-rotation fields post-storage compared with post-harvest values; the severity scores were higher as well. The changes in values exhibited after storage are consistent with typical storage behavior.