Journal of the NACAA
Volume 4, Issue 2 - November, 2011
The Relative Physiology and Phenology Of Cucurbit Yellow Stunting Disorder Virus (CYSDV) In Desert Grown Muskmelon (Cucumis melo, var. reticulatus)
- Kurt D. Nolte, Agricultural Extension Agent, University of Arizona, Yuma County Cooperative Extension
John C. Palumbo, Research Entomologist / Extension Specialist, University of Arizona, Department of Entomology
Stacey R. Bealmear, Urban Horticultural Extension Agent, University of Arizona, Yuma County Cooperative Extension
Muskmelon was grown using production practices that encouraged the development of Cucurbit yellow stunting disorder virus (CYSDV). The progression of CYSDV infection was examined through the season on different tissue types [leaves, vines and fruits] and the effect of CYSDV infection on physiological functions was examined at two harvest dates. CYSDV showed a slow rate of infection at the beginning of the season and a greater percentage of infection of leaves at the basal end of vines. Photosynthesis was reduced in virus-affected infected plants by an average of 60% at the mid-season harvest compared with symptom reduced plants. Leaf tissue with symptoms had lower photosynthetic rates than healthy leaves. One of the most marked differences in photosynthesis was between leaves with and without symptoms and between plants in different disease severity categories. At 47 DAP, leaves with fewer symptoms had significantly higher rates of photosynthesis than leaves with symptoms, since the latter are likely to have greater viral loads and, consequently, to have greater viral damage to the photosynthetic apparatus. The ability to maintain high assimilation physiology in the presence of the virus may help cultivars withstand CYSDV infection and maintain final yields.
Keywords: Cantaloupe, muskmelon, CYSDV, Cucurbit Yellow Stunting Disorder Virus, gas exchange analysis, photosynthesis, transpiration, tolerance, water-use efficiency
Plant virus infections are known to affect plant physiology dramatically, including decreased photosynthesis, increased respiration and altered carbohydrate levels (Ryslava et al., 2003). The alteration of these physiological processes caused by viral diseases is one of the primary causes of decreased crop productivity across the world (Agrios, 1997). Much of the research concerning plant viruses, however, has been focused on determining the structure, genetics, transport and localization of viruses in plant tissues, with much less effort aimed at understanding the effect of viral infection on host plant physiology, growth and overall productivity (Arias et al., 2003).
CYSDV is a whitefly vectored virus that now affects melon and other cucurbit production in North America (Kao et al., 2000). CYSDV produces initial symptoms of severe interveinal chlorosis and green spots on oldest leaves. Spots appear between 14 and 22 days post inoculation and definite symptoms are visible after 30 days (Celix et al., 1996). Severe symptoms (Fig. 1) include complete leaf lamina yellowing (except for veins) and leaf rolling and brittleness has been described (Celix et al., 1996). Since Criniviruses produce symptoms mainly in older leaves, CYSDV symptoms may be easily confused with physiological disorders, nutritional deficiencies, inadequate water, insect damage, natural senescence, or pesticide damage (Wisler et al., 1998).
Figure 1. Muskmelon leaves expressing severe symptoms of Cucurbit yellow stunting disorder virus (CYSDV).
Currently, there are no known muskmelon cultivars with complete resistance, although melon germ plasm resistant to CYSDV has been reported (Marco et al., 2003). And, as with many traits, it has been suggested that CYSDV resistance could vary between melon cultivars, as well as the associated physiological and other growth related responses (Eid et al., 2006). Because of the lack of a significant cultural and chemical means to affect the severity of CYSDV outbreaks, future cultivars that suppress epidemics of CYSDV will remain the most important tools in minimizing yield loss in melon production (Berdiales et al., 1999). The objectives of this study were: (i) to follow the progression of CYSDV infection through different muskmelon tissue types; (ii) to determine the physiological effects of CYSDV infection through time in leaf tissue with and without symptoms; and (iii) to establish whether differences between muskmelon cultivars in physiological properties relate to viral infection.
On 15 August, 2008, 29 common desert grown muskmelon cultivars were hand-planted in replicated plots (n=3/cultivar) located at the University of Arizona, Yuma Agricultural Center, Yuma, AZ, USA. The planting date was chosen to maximize the incidence and severity of CYSDV. Muskmelon seeds were planted in beds (25 ft x 7 ft), with a 12 in seed spacing. Prior to planting, plots were fertilized with 11-52-0 (N:P:K) at a rate of 500 lbs/a followed by liquid injection applications of 32-0-0 (N:P:K), 21 days after planting (DAP) at a rate of 20 gal/a. Plants were furrow irrigated adequately to avoid any symptoms of water stress and, to encourage CYSDV infection, whitefly control (Assail and Capture at 5 oz/a) was used only when immaturepopulations were determined to limit plant growth.
The significance of CYSDV on muskmelon growth and development was appraised on 7 randomly selected plants from each plot at 26 and 47 DAP. For these assessments, vine length and node number was determined and the number of leaves, flowers and fruits and whole plant dry weights were quantified. Nondestructive leaf chlorophyll content was estimated and used as an indirect indicator of CYSDV infection. Chlorophyll index values (SPAD) were taken on the fifth (leaf position #5) and tenth (leaf position #10) nodal, basal leaf with a Minolta SPAD-502 chlorophyll meter (Minolta Konica Co. Ltd., Japan). Chlorophyll indexing was determined to be closely correlated with visual symptoms: > 50, uniform green; 50-40, mottled light green; 40-30, interveinal chlorosis; <30, whole leaf yellowing. Two asymptomatic (cvs. Trinity, Navigator) and two symptomatic cultivars (cvs. Olympic Gold, Laser) were identified and tagged for later gas exchange testing.
At fruit maturity, disease severity was assessed in leaves using a five-level Likert scale (Leaf Severity Score: 1, no symptoms; 2, light green or speckling; 3, light green overall with lighter green interveins; 4, interveinal yellowing; 5, yellow leaf). After immediately scoring for severity, plots were harvested and total yield, fruit size, and soluble solids (°Brix) was determined.
To evaluate the physiological effects of CYSDV in muskmelon plants, gas exchange measurements were made at 26 and 40 DAP in cultivars showing limited expression of CYSDV (cvs. Trinity and Navigator) and those that had severe CYSDV symptoms (cvs. Olympic Gold and Laser). To standardize the analysis, the sixth nodal, apical leaf from each of 15 previously tagged plants per cultivar was analyzed for photosynthetic and transpiration rates, and water-use efficiency, calculated as the ratio of photosynthesis to transpiration (Ehleringer & Osmond, 1989). Gas exchange was measured at 2000 mu mol photons m-2s-1as described by Knee and Thomas (2002) using a gas exchange analytical system (Qubit; Hamilton, Ontario, Canada) between the hours of 09·00 and 13·00 when photosynthetic levels were maximal (data not shown).
Statistical analyses were performed using SAS (Statistical Analysis System Institute, Inc.). Fisher’s LSD means separation tests were used to determine if CYSDV severity was significantly affected by cultivar in different tissue types, growth parameters, gas exchange, and other physiological traits as a function of harvest date.
Relative CYSDV effects on muskmelon quality and yield
A fruit yield and quality comparison among 29 common desert muskmelon cultivars from 9 source companies determined that the sensitivity of muskmelon cultivars to CYSDV is diverse and wide ranging (Table 1). All cultivars evaluated were vulnerable to the effects of CYSDV with leaf symptoms ranging from minor interveinal yellowing (severity score, 2.0) to severe yellowing and chronic CYSDV effects (severity score, 5.0). Many cultivars displayed varying degrees of CYSDV incidence with some cultivars showing a decrease in disease prevalence within plots. While most cultivars displayed significant effects of CYSDV infection in terms of overall yield, size and sugar contents, the cultivars, Navigator, Caribbean Gold and Trinity showed a reduction in CYSDV sensitivity while sustaining fruit yield and quality. The majority of cultivars, including Olympic Gold and Laser, typically displayed significant effects of CYSDV and, had lower overall yields, inferior market sizes and decreased average fruit weights and, in general, reduced sugar contents (Table 1). The results suggest that the rate of CYSDV progression in muskmelon appears to be dependent on the genetic makeup of the cultivar, as suggested by Marco et al., 2003.
The progress of CYSDV in susceptible muskmelon cultivars
For the purpose of comparing the relative physiological effects of muskmelon infected with CYSDV, 2 cultivars which displayed reduced CYSDV effects (Trinity and Navigator) were compared with 2 cultivars that were more susceptible to the disease (Olympic Gold and Laser) (Table 1). Incidence of CYSDV disease in muskmelon plants was followed from early plant development [10 September (26 DAP)] to mid-season establishment [1 October (47 DAP)] during the fall when CYSDV infection rates are high. The presence of CYSDV in muskmelon cultivars was confirmed by real-time quantitative PCR using crude RNA extracts (data not shown).
There were no significant differences among cultivars at 26 DAP for the incidence of CYSDV measured in either whole plants (leaves, flowers, plant weight and fruit number) or in primary and secondary vines (length, node numbers) at 26 DAP (Table 2). However, there was a greater percentage of older, position 5 leaves with CYSDV symptoms than in younger , position 10 leaves when measured by SPAD chlorophyll indexing (d.f. = 2, LSD = 3.4, P-value = 0.0425). It appeared that infection progressed from the plant base acropetally towards the apex of the plant. SPAD chlorophyll indexing suggested that older leaves expressed the severity of the disease before the onset of the disease at the meristematic portions of the plant (Fig. 2).
Significantly higher levels of disease symptoms at the whole plant level occurred at 47 DAP. Leaf, flower and fruit numbers and plant dry weights were significantly lower in muskmelon cultivars sensitive to CYSDV. These plants also had shorter primary and secondary vines and a reduced number of nodes per vine (Table 2). In CYSDV susceptible cultivars, leaf chlorosis was significantly higher at the basal region of vines, while chlorophyll contents from the apical leaves were similar among all muskmelon cultivars examined (Fig. 2). The tendency of CYSDV expression in susceptible cultivars, particularly older leaves, and the absence of significant growth or developmental differences at 47 DAP (Table 2), suggests that CYSDV affects plant physiology and growth later in the season. Moreover, all plants survived the entire season, supporting a general pattern of late-season effects characterized by higher disease incidence and severity.
Figure 2. The influence of CYSDV infection on the relative chlorophyll contents (SPAD index) of muskmelon leaves with reduced CYSDV symptoms (Navigator, 47 DAP), and with CYSDV symptoms (Olympic Gold, 26 and 47 DAP).
CYSDV influence on gas exchange
Gas exchange was significantly affected by CYSDV infection at later growth stages of muskmelon. When gas exchange was evaluated in symptomless leaves and those with a wide range of CYSDV symptoms, rates differed by as much as 2.5 fold (Fig. 3). And, more specifically in CYSDV susceptible cultivars, the disease significantly decreased photosynthesis, transpiration and water use efficiency at 40 DAP. The actual percentage decrease of gas exchange was quite large as a result of infection and appeared to be associated with leaf chlorophyll content. The decrease in photosynthesis was as high as 60% for cvs. Olympic Gold and Laser, while transpiration for these cultivars differed by approximately 30% at 40 DAP. In addition, leaves with CYSDV symptoms were thicker (data not shown) than symptomless leaves, a result that agrees with Swiech et al. (2001), who found thicker leaves in virus-infected sugar beet plants because of a general enlargement of mesophyll cells. This cell enlargement may decrease photosynthesis by interfering with light interception and diffusion of CO2into the leaf (Swiech et al., 2001). Water-use efficiency also seemed to be influenced by cultivar and disease severity such that highly infected plants tested had lower water-use efficiency than those with less CYSDV symptoms. Disease severity had a significant effect on photosynthesis and water-use efficiency, with CYSDV tolerant plants exhibiting a significantly higher photosynthetic capacity and higher water-use efficiency than the more severely infected cultivars (Fig. 3).
Figure 3. Photosynthesis (a), transpiration (b) and water-use efficiency (WUE) (c) in CYSDV infected leaves, 26 days after planting (black bars) and 40 days after planting (white bars) of 4 muskmelon cultivars. Error bars indicate standard error of the mean (n = 15). Means followed by the same letter are not significantly different according to a Fisher’s LSD means separation test.
Progress of CYSDV symptoms in the monitored crops was slow at early plant growth stages, with viral effects being detected in plants as the season advanced. Similarly, Walters et al. (2005) found that the onset of Watermelon mosaic virus was slow and infected plants expressed decreased photosynthesis and chlorophyll content at later stages of growth. As reported here, decreased photosynthetic rate by CYSDV of roughly 60% reflect values recorded in other studies (Clover et al., 1999; Swiech et al., 2001) and was of greater magnitude later in the season. Reductions in photosynthesis in response to viral infection reported here are similar to other studies in which viral infection caused the simultaneous effects on gas exchange, and leaf chlorosis (Bertamini et al., 2004), perhaps reflecting the pattern of a more severe infection response during fruiting, or the imposed or compounding effects of plant age or senescence.
This study identified a possible CYSDV tolerance mechanism that is physiologically based: the ability to maintain near-normal photosynthetic levels in symptomless tissue, even in the presence of viral infection. Lyerly et al. (2002) suggested that this type of suppressive mechanism, present in some muskmelon cultivars, was not resistance to CYSDV per se. The ability to maintain high physiological function in the presence of CYSDV may be a more important resistance or tolerance mechanism than actual avoidance of infection because of the near omnipresence of CYSDV in muskmelon producing regions of the desert southwest. The information presented in the current study is important because a better understanding of the mechanisms behind the viral impact on host plant physiology can lead to the development of improved cultivars that either resist viral infection or can better tolerate infection by experiencing less severe symptoms (Balachandran et al., 1997).
We thank Tony Tellez, Gerardo Villegas, and Judy Brown for making this laboratory and field work possible.
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