Effects of Monosporascus cannonballus on Root Traits of Susceptible and Tolerant Melon (Cucumis melo L.) Cultivars

Kevin Crosby

Department of Horticulture, Texas A&M University, College Station, TX 77843

David Wolff

Sakata Seeds America, P.O. Box 1118, Lehigh, FL 33970-1118

Additional index words. Monosporascus cannonballus, cantaloupe, muskmelon, ascospores

Abstract. The fungus Monosporascus cannonballus attacks melon roots and causes root rot/vine decline disease. This has reduced productivity of commercial muskmelon and honeydew cultivars in South Texas. The current research focuses on the extent of damage caused by this pathogen to various melon cultivars representing both susceptible and tolerant genotypes. In order to assess the impact of the fungus on several melon root traits, a greenhouse experiment was carried out to compare inoculated and healthy root systems. The four cultivars, 'Magnum 45', 'Caravelle', 'Deltex', and 'Doublon' were grown in a sand medium for 30 days during June, 1998. The treatment consisted of inoculating sand with 60 colony forming units per gram with the severe Monosporascus strain, TX90-25. The design was a randomized complete block with seven replications. The high temperatures(24 °C night and 38 °C day) favored fungal development. The control and inoculated root systems were carefully cleaned and stored in plastic bags at 4 °C while being evaluated. Roots were scanned into a computer and analyzed by the Rhizo Pro 3.8 program. The traits of interest included total root length, average root diameter, number of root tips, number of fine roots (0 to 0.5 mm) and number of small roots (0.5 to 1 mm). Significant differences existed between the two tolerant cultivars, 'Deltex' and 'Doublon' and the two susceptible cultivars, 'Magnum' and 'Caravelle' for four of the traits. 'Deltex' exhibited significantly greater root length, fine and small root length and root tip number than both susceptible cultivars. 'Doublon' exhibited significantly greater root length, fine and small root length and root tip number than 'Caravelle.' The results suggest the likely existence of genetic components for tolerance and possibly root vigor.

The Rio Grande Valley of South Texas is an important production region for honeydew and cantaloupe melons (Cucumis melo. L). The subtropical climate in this region allows early planting and the earliest harvest in the continental U.S. The concomitant profitability of the crop has helped it to survive the economic pressures which have reduced or eliminated production of other vegetables in the region.

The greatest threat to the melon production has been losses due to vine decline diseases. Numerous fungi attack roots, stems and fruit causing crop loss. One of the most devastating diseases in recent years is root rot/vine decline caused by Monosporascus cannonballus (Martyn and Miller, 1996). This fungus inhabits the calcareous soils in South Texas and other hot, dry regions. Distribution is widespread in cultivated soils and as

cospores are extremely heat and drought tolerant (Stanghellini et al., 1996). It causes necrosis on roots at all stages of development (Mertely et al., 1991) Root loss leads to excessive stress on the vines and decline or collapse. This problem is exacerbated by fruit load, heat, drought and physiological stresses (Wolff, 1996). Replicated field trials and germplasm screening have identified several cultivars with tolerance as well as highly susceptible ones (Wolff and Miller, 1997; Cohen et al., 1996). Many other cultivars fall into an intermediate category but none exhibit immunity. In addition to field tests, several greenhouse experiments have been conducted with different strains of M. cannonballus and high inoculum levels (Wolff and Miller, 1997). In this process of screening for resistant genotypes, none have exhibited immunity to the fungus but several have proven to be tolerant.

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The purpose of this experiment was to identify the effects of severe M. cannonballus infections on various root characters in melons. The possibility of breeding for improved root structures to increase disease tolerance is a major goal. Another is to study the genetic control of root trait development in a diverse sample of melon genotypes. Prior to conducting genetic analyses, the existence of differences needs to be confirmed. This experiment provides evidence that differences in root structure exist between tolerant and susceptible melon genotypes. These differences are examined in relation to root damage caused by M. cannonballus.

Materials and methods

Four cultivars of melons were selected based on previous field evaluations for tolerance to M. cannonballus. 'Deltex', an Ananas type, and 'Doublon', a 'Charentais' type, both exhibited tolerance to the fungus in field and greenhouse tests. 'Magnum 45', a Western shipping muskmelon type, is a standard susceptible check used in many trials. 'Caravelle', also a muskmelon, is a popular commercial hybrid which exhibits extreme susceptibility to the disease.

Before sowing the seed, the sand media was pasteurized at 160 oF (71.1 oC) for 10 h to kill other potential pathogens. The sand pH was 8.5 to 9.0, which was favorable for M. cannonballus growth. Three-gallon (11.4 L), black, plastic pots were filled with the sand and inoculated with 60 colony forming units (CFUs) of M. cannonballus per gram of media. The virulent M. cannonballus strain, TX90-25, was used. The CFUs were measured by a standard dilution and colony count on petri plates containing PDA with no streptomycin (Mertely et al., 1993). One ounce of Osmocote was

mixed into the top few inches of sand in each pot. The pots were then sown with seed from the four cultivars and arranged in a randomized complete block design with seven replications. Two treatments, inoculated and control, were grown side by side in the greenhouse. All seeds were sown on 25 June 1998. Germination was examined each day and the growth period begun accordingly. All plants were grown for 30 days. Plants were hand watered with one liter every 3 or 4 days based on temperature and sand moisture. In addition, each plant received 600 mL of Peters 20­20­20, 400 ppm, plus micronutrients every 7 days through the growth period. All plants were harvested in a 24-h period. After cutting the vine and removing the pot each root system was carefully submerged into a 50 gal (189.3 L) drum of water inside a fine mesh cage. This allowed all the sand to flow out, leaving only the roots. Entire root systems and all fragments were carefully blotted dry on paper towels and placed in Ziploc plastic bags inside a 4 °C refrigerator.

Over the next 6 days, each root system was carefully placed on a clear glass plate and submerged in a thin layer of water. After all roots were evenly spread apart, the plate was placed on a HP Scanjet 4c wide scanner. The roots were scanned into the computer program, Rhizo 3.8 Pro, by Regent Instruments. After all 56 root systems had been scanned, analysis by the program was initiated. The program generates values for total root length, average root diameter, number of root tips, and lengths of roots in specific diameter classes. All data were input into SAS for windows, statistical analysis program. ANOVA and mean separation using the GLM and Means/LSD commands were generated for all root traits examined.

Table 1. Analysis of variance (mean squares) for root traits in four melon cultivars inoculated with M. cannonballus, and their control treatments.z

Total Mean Length (mm)

Source df length (cm) diam (cm) Root tips 0<L<0.5 0.5<L<1.0

Cultivar (C) 3 1771408 0.003005** 15395244** 952795* 39328

Treatment (T) 1 90200201** 0.007859** 706102146** 44715837** 2039793**

Replication 6 1046270 0.000198 5051314 469284 33341

C ¥ T 3 1997604* 0.003008** 19712361** 1222239* 46183

*,**Significant at P = 0.05 and 0.01, respectively.

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The degree of these interaction effects was revealed by the mean separation analyses, comparing cultivars within each treatment. For the controls, few significant differences existed among the four cultivars for the five root traits (Table 2). The exceptions include significantly lower total root length and fine root length for 'Doublon' as compared to 'Deltex.' In addition, 'Doublon', exhibited significantly less root tips and fine root length than 'Magnum.'

The treated roots exhibited many more significant differences (Table 3). 'Deltex' exhibited significantly greater root length, number of root tips, fine root length (0 to 0.5 mm), and small root length (0.5 to 1.0 mm) than 'Caravelle' and 'Magnum.' 'Doublon' was significantly greater than 'Caravelle' but not 'Magnum' for the same traits. 'Magnum' was not significantly higher than 'Caravelle' for the same four traits. 'Caravelle' was significantly higher in average root diameter than the other three cultivars.

These results are interesting in light of the field evaluations for tolerance to M. cannonballus. Few significant differences for the root traits examined in healthy roots are somewhat surprising. The higher mean root length and fine root values for

Figure 1. Control (c) and Monosporascus (TX90-25)-inoculated (i) melon roots of susceptible 'Caravelle' and tolerant 'Deltex'. (A) 'Caravelle' (c), (B) 'Caravelle' (i), (C) 'Deltex' (c), and (D) 'Deltex' (i).

Results and discussion

The initial ANOVA (Table 1) revealed significant or highly significant differences among the four cultivars for average root diameter, number of root tips and length of fine roots (0 to 0.5 mm). Highly significant differences existed between treated and control roots for every root trait (Figure 1). There were, however, significant interaction effects between cultivar and treatment for four of the five traitstotal root length, average root diameter, number of root tips and length of fine roots (0 to 0.5 mm).

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Table 2. Separation of root trait means among control plants of four cultivars.

Total Mean Length (mm)

Cultivar length (cm) diam (cm) Root tips 0<L<0.5 0.5<L<1.0

Deltex 4203.9 az 0.0312 bc 11755 ab 3116.9 a 641.3 a

Magnum 4051.1 ab 0.0295 c 12485 a 3165.0 a 583.8 a

Caravelle 3750.8 ab 0.0329 ab 10158 bc 2813.0 ab 609.3 a

Doublon 3085.3 b 0.0351 a 9093 c 2356.2 b 509.5 a

zMeans with the same letter are not significantly different.

Table 3. Separation of root trait means among four cultivars inoculated with M. cannonballus.

Total Mean Length (mm)

Cultivar length (cm) diam (cm) Root tips 0<L<0.5 0.5<L<1.0

Deltex 1817.9 az 0.0352 b 5352 a 1535.6 a 302.7 a

Doublon 1554.4 ab 0.0367 b 4937 ab 1306.3 ab 267.8 ab

Magnum 901.5 bc 0.0537 b 2822 bc 813.9 bc 136.6 bc

Caravelle 664.1 c 0.0978 a 1973 c 646.8 c 110.0 c

zMeans with the same letter are not significantly different.

'Deltex' may not be significant in this relatively small data set. Future experiments with larger numbers of roots may reveal more differences. Nonetheless, the significant differences among the cultivars for the root traits in inoculated roots were promising. The visible difference between 'Deltex' and 'Caravelle' is obvious in Figure 1. The superior performance of 'Deltex' and 'Doublon' root systems corresponds well to the field tolerance observed in soils infested with M. cannonballus. The reduced damage compared to 'Caravelle' and 'Magnum' suggests the existence of some resistance mechanism. Whether this mechanism involves chemical or physiological responses to infection or just increased root vigor remains to be seen. Research to evaluate the genetic control of this resistance and the root traits examined is currently underway. The evaluation of heritability for these traits and their impact on disease tolerance will be an important step in developing new cultivars able to withstand infection by M. cannonballus.

Literature cited

Cohen, R., Y. Elkind, Y. Burger, R. Offenbach, and H. Nerson. 1996. Variation in the response of melon geno

types to sudden wilt. Euphytica 87:91­95.

Martyn, R.D. and M.E. Miller. 1996. Monosporascus root rot and vine decline: An emerging disease of melons worldwide. Plant Dis. 80:716­725.

Mertely, J.C., R.D. Martyn, M.E. Miller, and B.D. Bruton. 1991. Role of Monosporascus cannonballus and other fungi in a root rot/vine decline disease of muskmelon. Plant Dis. 75:1133­1137.

Mertely, J.C., R.D. Martyn, M.E. Miller, and B.D. Bruton. 1993. Quantification of Monosporascus cannonballus ascospores in commercial muskmelon fields in south Texas. Plant Dis. 77:766­771.

Stanghellini, M.E., D.H. Kim, and S.L. Rasmussen. 1996. Ascospores of Monosporascus cannonballus: germination and distribution in cultivated and desert soils in Arizona. Phytopathology 86:509­514.

Wolff, D.W. 1996. Genotype, fruit load and temperature affect Monosporascus root rot/vine decline symptom expression in melon (Cucumis melo L.). Melon production systems in south Texas. Texas A&M Univ. System, Agr. Res. and Ext. Ctr., Weslaco.

Wolff, D.W. and M.E. Miller. 1997. Evaluation of melon plant introductions for resistance to Monosporascus cannonballus. Melon production systems in south Texas. Texas A&M Univ. System, Agr. Res. and Ext. Ctr., Weslaco.

Wolff, D.W. and M.E. Miller. 1997. Evaluation of MRRVD field tolerant lines and hybrids in the field and greenhouse. Melon production systems in south Texas. Texas A&M Univ. System, Agr. Res. and Ext. Ctr., Weslaco.

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