There are numerous above ground (foliar) and below ground (soil-borne) diseases
of potato that have been reported as constraints to productivity in diverse
agro-ecological zones of the world (Powelson et al.,
1993). The disease symptoms often affect various above-ground and below
ground parts of potato plants. Foliar diseases such as early blight, late blight
and potato viruses are often reported to be more problematic, due to their devastating
effects on reductions of photosynthetic leaf area and subsequent defoliation,
resulting in tremendous yield loss. Potato diseases that affect below-ground
plant parts (roots, stolons and tubers) have previously been less investigated
due to the complexity of interactive factors affecting potato plants and tubers.
However, further studies and documentation of the biological and edaphic factors
that contribute to development of tuber-borne diseases have elucidated the role
of these diseases.
One of the major factors contributing to renewed interest in quantifying and
assessing soil-borne or potato tuber diseases is the increases in consumption
and utilization of potato as table stock or wares, in both developed and developing
countries (Scott et al., 2000). The increase
in consumption and utilization of potato has indirectly influenced industry
and consumer considerations of tuber quality especially as they relate to quality
appearance and palatability of potato tubers. Similarly, potato tubers shipped
for processing at industrial outlets have also been subjected to quality assessment
(external appearance, external defects) and documentation of characteristics
such as tuber diseases.
There are many tuber-borne diseases of potato which have been previously reported.
Among these are tuber black scurf (Rhizoctonia solani Kuhn), black dot
(Colletotricchum coccodes (Wallr.) S.J. Hughes), silver scurf (Helminthosporium
solani Durieu and Mont. (Syn.) Spondylocladium atrovirens Harz.)
and common scab (Streptomyces scabei) which are the most common tuber
diseases and are known to occur in various potato production regions of the
world (Adams et al., 1987; Adams
and Stevenson, 1990). The potential effects of these diseases on tuber quality,
survival, as well as their inoculum potential are of considerable importance
and have been the primary justification for studying these diseases.
The research findings from previous studies on tuber diseases have focused
primarily on pathogen prevalence of various diseases in diverse agro-ecosystems,
as well as methods for pathogen controls or disease management (Carling
et al., 1989; Johnson and Miliczky, 1993;
Larkin and Honeycutt, 2006). Potato cultivars, soil
amendments and crop rotations have been extensively examined as management options
for potato tuber disease control (Olanya et al.,
2006; Peters et al., 2004; Honeycutt
et al., 1996). In the above studies, the interactive or complementary
effects of irrigation were not investigated. Similarly, the role of fungicides
and other chemical compounds for management of foliar and seed-borne, or tuber-borne
potato diseases have also been examined.
Among the most important edaphic or environmental factors that influence pathogen
infection and disease development on potato and other agronomic crops is moisture
or relative humidity. Water management has been shown to provide humid conditions
or microclimate favorable for pathogen infection and development of certain
diseases (Rotem et al., 1970; Olanya
et al., 2007a, b). In contrast to the detrimental
indirect effects of irrigation mentioned above, the positive attributes of water
management is linked to the provision of adequate soil moisture for potato growth
at critical duration of the cropping seasons (Rotem and
Palti, 1969; Lapwood et al., 1973;
Starr et al., 2008). In many other previous research investigations
and findings, the occurrences and incidences of tuber-borne diseases have often
been examined on a single disease scenario or treatment effect, rather than
the interactive effect of various treatment factors on multiple potato diseases,
or their development and management in storage environments.
In this study, we examine the effects of supplemental irrigation on the incidence
of potato tuber diseases and evaluate the effects of selected soil amendments
and crop rotations and their interactive effects on potato tuber diseases This
research supplements a previous study conducted during the same time periods,
documenting the effects of supplemental irrigation and cultivar effects on tuber-borne
potato diseases, in which similar methodology were used even though experimental
treatments differed (Olanya et al., 2010).
MATERIALS AND METHODS
Site description and plot establishment: The experimental plots at the
University of Maine, Aroostook Research Farm in Presque Isle, Maine; were used
for the study from 1994 to 1997. The soil at the experimental site is a Caribou
gravelly loam (fine-loamy, mixed, frigid isotic Haplorthods). Average air temperature
and monthly rainfall totals were the same as previously described (Olanya
et al., 2010).
Irrigation treatments and potato diseases (1994 to 1995 experiments): Irrigation experiments were examined during the two years. The experiment assessed the effects of supplemental irrigation, soil amendments and crop rotation on potato tuber diseases, The experiments were arranged in split-split plot designs consisting of three irrigation treatments (best, reduced and non-irrigated) during the 1994 to 1995 years. The main plots were irrigation treatments with plot dimensions of 27.2x27.2 m. The sub-plots (9.1x13 m) consisted of two soil amendments and two crop rotations (sub-sub plot) within the main plots and a single potato cultivar (superior) was used. In the irrigated treatments, supplemental irrigation and subsequent scheduling of irrigation events were initiated and conducted based on gravimetric soil water content and gypsum blocks readings in 1994 and 1995. There were four replications per experimental treatment.
The irrigation system used for the experiment was overhead sprinklers with plastic pipes. Buffer zones (3 to 5 m wide) were maintained around each plot to isolate irrigation treatments and minimize interplot interference with water applications. In the best irrigation treatment, water was applied to maintain optimum plant and soil moisture availability, based on gypsum blocks and tensiometer readings. In reduced irrigation treatment, water was applied to field plots from tuber initiation to mid-bulking with a goal of maintaining 50% Plant Available Water (PAW). A non-irrigated control or check had no water applied to field plots. In 1994, the initial irrigation application was started on July 18 and final application was on August 20. In 1995, initial and final irrigation applications were on July 3 and August 28, respectively.
Irrigation treatments and potato diseases (1996 to 1997 experiments): From 1996 to 1997, the experiments were conducted to assess the effects of supplemental irrigation, soil amendments and crop rotations on potato tuber diseases. Two irrigation treatments (irrigated and un-irrigated) were used as the main plots. The water applications consisted of: irrigation at 35% soil moisture depletion where water was applied at 1.42 cm per application and the un irrigated control (check) where no water was applied to field plots. The experiments were arranged in split-split plot designs consisting of the two irrigation treatments, two soil amendments and two crop rotations. The main plots were irrigation treatments with plot dimensions of 27.2x27.2 m. The sub-plots (9.1x13 m) consisted of two soil amendments and two crop rotations (sub-sub plots) within the main plots. In 1996 and 1997, tensiometer readings were used to determine soil water content for scheduling irrigation events. The cultivar Superior was planted in all experiments.
Crop rotation and potato diseases (1994 to 1997 experiments): From 1994 to 1997, two crop rotations were used. Crop rotation treatments consisted of: (1) small grains (oats cv. Porter) and, (2) green manure crop (mixture of oats, peas and vetch). The small grains crop was seeded at the rate of 201.78 kg ha-1 and the green manure crops (oats, peas, vetch) were seeded at the rates of 53.8, 168.15, 33.63 kg ha-1 and broadcast as mixtures; respectively. After maturity, the crops were subsequently plowed in the soil in the fall in each year.
Soil amendment and potato diseases (1994 to 1997 experiments): During the 1994 to 1995 experiments, the two soil amendment treatments consisted of: (1) small grain + manure + compost (Grainmc) and (2) green manure + manure + compost (Greenmc). Straw from small grain crop (oats) and green manure crop (peas vetch) was plowed under field soil while manure and compost were applied to the soil surface at the rates of 44.84 and 22.42 metric t ha-1, respectively; and then disked. The compost contained mixture of waste potato tubers, saw dust, wood ash. The manure amendment consisted of cow dung which was obtained from beef cattle. In the 1996 to 1997 experiments, two soil amendments which differed from that of 1994 to 1995 were used. The soil treatments were: (1) amended by the addition of manure + compost and consisted of the same constituents as described above, or (2) non-amended treatment. The application rates for soil amendments with manure and compost during 1996 and 1997 were 44.84 and 22.42 metric t ha-1.
Foliar disease control: Foliar diseases in field plots were controlled by the application of foliar fungicides namely mancozeb during 1994 and 1995. In 1996 to 1997, foliar disease management was accomplished by the application of chlorothalonil.
Disease assessment: The incidence of various tuber diseases such as:
black scurf, silver scurf, black dot and common scab were assessed visually
after harvest on a sample size of 200 tubers per treatment (James,
1971). The sampling methods and storage conditions were the same as those
previously described (Olanya et al., 2010). The
occurrence of black dot and silver scurf on potato tubers were differentiated
based on observation of colletotrichum under dissecting scope immediately after
washing of tuber samples. The presence of silver scurf was verified by microscopic
observation of spores of H. solani obtained on two-sided tape. The potato
tubers with black scurf and containing symptoms characteristic of R. solani
sclerotia, as well as common scab were also identified. Potato tubers with and
without symptoms was counted from a sample lot and the incidence of tuber diseases
on potato tubers were then determined (Number of tubers diseased/Total number
of tubers assessedx100).
Data analyses: Prior to the analysis, percent disease incidence, data were subjected to tests for normality of variances using the Shapiro-Wilk test of residuals (SAS Institute, Cary, NC, USA). Where assumptions (additivity, constancy of variance, normality) were not met, data transformation was accomplished prior to analysis of variance of the percentage disease incidence data. To determine the significance of treatment effects, the percentages of black scurf, silver scurf, black dot and common scab incidences were analyzed for each disease separately during each cropping year using the GLM procedures of SAS (SAS Institute, Cary, NC, USA). Where significant treatment effects were observed, means were separated using the least significant difference (p = 0.05). The average disease incidences (%) were also shown graphically to illustrate the interaction of some tuber diseases and treatment effects.
Irrigation effects on potato tuber diseases: The irrigation effects
were significant (p<0.05) for tuber diseases consisting of black scurf and
silver scurf in 1994 and for black scurf only in 1995 (Fig. 1a,
b). The irrigation application significantly (p<0.05) affected the incidence
of silver scurf and common scab diseases during 1997 (Table 1,
||Effect of supplemental irrigation on the percent incidence
of tuber diseases on Superior cultivar. The irrigation treatments refer
to best (available optimum plant and soil water), reduced (water applied
from tuber initiation to mid-bulking and maintained at 50% PAW) and un-irrigated
(check). Data represent average disease levels and different lower case
letters for each disease among irrigation treatments indicate significant
differences (p<0.05) based on Fishers LSD statistics. (a) 1994
and (b) 1995
The average incidence of tuber diseases was 0.3, 1.7, 13.9 and 19.3% for common
scab, silver scurf, black scurf and black dot, respectively; under un-irrigated
plots. In the irrigated treatments tuber disease levels were 14.6, 9.3, 13.7
and 16.5% for the same diseases, respectively.
Crop rotation effects on potato tuber diseases: Crop rotation treatments did not significantly affect the incidence of silver scurf during 1994 and 1995 cropping years (Table 1). Crop rotation significantly impacted silver scurf and black dot disease incidences in 1997 (Table 1).
||Effect of irrigation treatment on the percent incidence of
tuber diseases during the 1997 cropping year. Field plots were either irrigated,
or not irrigated and cropped to potato cultivar Superior. Different lower
case letters for each disease between irrigation treatments indicate significant
differences (p<0.05) based on Fishers LSD statistics
The average incidence of black dot was 15.8 and 21% on tubers planted under green manure and small grain rotations, respectively. Mean disease incidence for silver scurf was 3.1 and 6.2% on tubers in which potato was planted in plots rotated with green manure (oats, peas and vetch) and small grain (oats) when data was averaged across irrigation treatments during the same year (Fig. 3). The mean incidence of black dot on tubers was significantly (p<0.05) greater on potato planted in plots where green manure (21.3%) was used as a rotation crop than on potato planted in plots rotated to small grains (12.4%) under irrigation. The difference in black dot incidence on tubers between the two rotations was also observed when the treatment was non-irrigated. The incidence of silver scurf disease on potato tubers obtained from green manure and small grain rotation plots was 5.2 and 12.8%, respectively (Fig. 3).
Soil amendment effects on potato tuber diseases: The addition of soil amendments did not significantly affect the incidence of black dot during the 1994 and 1995 cropping years. Soil amendment had significant (p<0.05) effects on black dot in 1996 and 1997 (Table 1). The incidence of black scurf disease in 1994 was 6% on potato tubers and significantly (p<0.05) higher in plots amended with grain + manure + compost compared to 0.8% on tubers harvested from plots amended with green manure+manure+compost (Fig. 4).
|| Analysis of variance on the effects of supplemental irrigation,
soil amendments and crop rotation on potato tuber diseases
|WIrrigation treatments consisted of un-irrigation,
best and reduced irrigation (1994-95); irrigated and un-irrigated (1996-97).
XSoil amendment treatments consisted of grain+manure+compost
and green manure+manure+compost (1994-1995). In 1996-1997, soil was amended
(manure+compost) or not amended. YThe rotations were small grain
(oats) and green manure crop (oats, peas and vetch). ZData could
not be analyzed due to very low or zero disease values. * and ** refer to
significant at p<0.05 and 01, respectively
||The effects of supplemental irrigation and crop rotation interactions
on percent incidence of tuber diseases in 1997. Crop rotation consisted
of green manure (vetch/peas/oats) or small grains (oats) and the plots were
either irrigated or had no water applied to field plots (un-irrigated check)
and planted with potato cultivar Superior . Different letters for each tuber
disease between rotation within irrigation treatment indicate significant
differences (p<0.05) based on Fishers LSD statistics
Irrigation by soil amendment effects on potato tuber diseases: The interaction
of irrigation treatment x soil amendment was significant for black scurf incidence
in 1994, where best irrigation treatment was used (Fig. 4)
and for silver scurf and black dot in 1997 (Fig. 5).
||The effects of soil amendment on percent incidence of black
scurf disease on potato cultivar Superior in 1994. Grainmc refer to amendment
of grain+manure+compost while Greenmc refer to Green manure crop (vetch/pea/oat)+
manure+compost added as amendments to field plots in 1994. Best irrigation
refers to water applied to maintain optimum plant and soil water availability,
based on gypsum blocks and tensiometer readings. In reduced irrigation,
water was applied to field plots from tuber initiation to mid-bulking with
a goal of maintaining 50% PAW. Un-irrigated or check treatment had no water
applied to field plots. Different letters for tuber disease between amendment
within irrigation treatment indicate significant differences (p<0.05)
based on Fishers LSD statistics
||The interaction effects of supplemental irrigation and soil
amendment (1997) on percent incidence of tuber diseases. Disease incidence
was quantified on the cultivar Superior. Amended plots had manure + compost
and non-amended plots had no amendments. Irrigated plots had water and un-irrigated
treatment had no water applied. Different letters for tuber disease between
amendments and within irrigation refer to significant differences in disease
incidence (p<0.05) based on Fishers LSD statistics
Black scurf incidence was significantly greater under amendment of grain+manure+compost
(grainmc) than on green manure+manure+compost amendment (greenmc). Silver scurf
incidence was consistently greater in amended compared to non-amended plots
under irrigated and un-irrigated treatments, while black dot was lower in amended
than non-amended plots irrespective of irrigation treatments (Fig.
Irrigation treatments had variable effects on soil-borne diseases across years
and treatments. This suggests that water application may enhance conditions
suitable for increases in the incidence of selective tuber diseases or not at
all. Previous research showed that lower temperatures and increased soil moisture
are favorable for stem canker infection (Hide and Firmager,
1989). Overhead irrigation experiments have been reported to alter potato
canopy microclimate and, thereby, affect disease development (Adams
and Stevenson, 1990). The variation in the incidence of black scurf disease
observed in this study implies that the timing and frequency of irrigation application
may influence soil moisture conditions conducive for pathogen infection and
The effects of irrigation treatments on black dot incidence on tubers varied
among irrigation treatments and years. This suggests that irrigation applications
may affect black dot development depending on the treatment and timing of water
application. In a previous research, water application early in the season was
noted to decrease infection of potato plants and tubers by the black dot pathogen
(Read and Hide, 1988). This was attributed to excessive
soil moisture for fungal growth and infection. Moderate soil moisture was also
noted to be conducive for black dot infection and disease development. Adams
and Stevenson (1990) also showed that excessive soil moisture may affect
potato tubers and lead to swollen lenticels and increased susceptibility to
tuber-borne infections. Additionally, overhead irrigation and microclimate have
been reported to affect black dot development on potato, depending on the initiation
of irrigation application relative to crop growth period (Raniere
and Crossan, 1959; Adams et al., 1987). In
this study, variation on irrigation effects on disease was noted, even though
black dot levels were higher than other diseases. Because we did not quantify
if there were any differences in pathogen density in soils in the various field
plots as well as possible variation in virulence among pathogen strains between
and within plots and years, it is difficult to mention what the precise effects
of tuber disease variation could be attributed to exactly. The effect of irrigation
treatments on plant diseases has also been attributed to conducive micro-climatic
conditions in the potato canopy leading to increased disease levels. Therefore,
the significance of the interactions of irrigation treatment x year for R.
solani suggests that treatment effects varied among irrigation applications
and years, perhaps due to microclimatic effects.
Crop rotation in the presence of irrigation applications did not significantly
affect the incidence of black scurf, except under best irrigation. The lack
of significant differences in the incidence of black scurf between the rotation
treatments under reduced and un-irrigated treatments suggests that the rotation
crops under investigation may be of limited use for R. solani control
under those conditions. It is also possible that due to the wide host-range
and saprophytic colonization of R. solani, fungal survival as sclerotia
on tubers and in soil, or other plants is possible. In this experiment, soil-borne
inoculum was not quantified, so it is possible that inoculum levels could have
been low for significant disease effects to occur under the above mentioned
irrigation treatments. This is in contrast to the field studies conducted by
Frank and Murphy (2001) and Larkin
and Honeycutt (2006); who observed that crop rotations or cover crops can
significantly affect Rhizoctonia diseases consistently in potato-based cropping
systems. The influence of cropping sequences in the absence of irrigation treatments
on soil-borne pathogen populations have also been demonstrated for Verticillium
dahliae and R. solani, in which certain rotations were more effective
for disease control, while others were not effective (Davis
and McDole, 1979; Larkin and Honeycutt, 2006).
Although differences in the incidence of black dot were observed among rotation
treatments under irrigation, these differences were significant only in some
years. This suggests that rotation crops may result in variable disease levels.
C. coccodes is a secondary pathogen and often associated with senescening
potato plants. Therefore, crop rotation and other methods designed to reduce
soil-borne inoculum of C. coccodes may have a limitation. Our findings
are in contrast to that of previous researchers. Scholte
et al. (1985) observed that the incidence of black dot and other
potato diseases were reduced in short rotation crops. Crop rotation has been
shown to reduce soil-borne inoculum due to non-host rotations (Larkin
and Honeycutt, 2006). However, because crop rotation was used in conjunction
with irrigation treatments, our results may differ from previous studies due
to the added moisture components. The pathogen can also be easily introduced
on seed tubers which could impact disease levels in subsequent cropping years
(Read and Hide, 1988).
Similarly, the effect of crop rotation resulted in differences in the incidence
of silver scurf on tubers in certain years. This suggests that green manure
(peas/vetch/oats) and small grain (oats) rotations may differentially impact
H. solani fungal propagules under irrigated experimental conditions.
Based on our results in which multiple pathogens were detected on potato tubers,
it is also possible that there may have been competition or antagonism between
H. solani and C. coccodes, or other pathogens since the presence
of one pathogen on tubers appears to limit the incidence of the other pathogen
on tubers. H. solani have been shown to have a limited host range and
impacted by other soil-borne pathogens (Errampalli et
Variation in tuber diseases was detected among soil amendments for selective
tuber diseases across years. This suggests addition of soil amendments may selectively
affect some of the tuber diseases examined. Soil amendments have been shown
to affect or decrease incidence and severity of soil-borne diseases due to increased
microbial activity (Larkin and Honeycutt, 2006). It
is likely that a similar mechanism may be a contributing factor for decreases
in the incidence of some tuber-borne diseases. The significant interactions
of soil amendments by irrigation on some tuber diseases imply that supplemental
irrigation may modify foliar or soil microclimate, which may be favorable or
detriment to some of the soil-borne pathogens.
We conclude that supplemental irrigation, crop rotation and addition of soil amendments affected the incidence of some potato tuber diseases. While crop rotation may reduce soil-borne propagule build-up by removal or reduction of susceptible host crops, soil amendment may enhance microbial competition or modify soil environment to the detriment of tuber pathogen or diseases. Disease increases may be aggravated by soil moisture conditions, but soil moisture may also be limiting for some tuber diseases. The timing and frequency of irrigation in relation to tuber initiation and tuber disease onset appears to be one of the factors determining tuber disease development. Although the pathogen strains, population density and virulence were not examined, those factors may influence development of potato tuber diseases. The frequent changes in pathogen composition and virulence over time, suggests that disease development and incidence may actually differ between years or from the period when assessment were made. Therefore, future studies should focus on these comparisons of management systems and their interactions in the light of changing pathogen composition or virulence and justifies the need for future investigations.
Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation for endorsement by the US Department of Agriculture or the University of Maine.
Authors thank Bart Bradbury, Jonathan Sisson and Anne Currier, previously of Aroostook Research Farm, Presque Ilse for technical support. This research was supported by Maine Agriculture and Forest Experiment Station (MAFES), the Maine Potato Board, U.S. Army Corp of Engineers, Aroostook Soil and Water Management Board. We thank MAFES and the USDA-ARS, New England Plant, Soil and Water Laboratory for their support.