Inoculum potential of verticillium—infested potato cultivars

Viable propagules ofVerticillium albo-atrum andV. dahliae were recovered from potato stems before and after burial in the field during 1968–1971. Of 14 cultivars tested: Red Pontiac and Superior ranked highest in numbers of propagules ofV. albo-atrum. Russet Burbank, Kennebec, and Irish Cobbler ranked highest withV. dahliae, respectively. Survival ofV. albo-atrum in infected stems was evident (5 ? 34 × 10³ ppg) even after burial in field soil for 17 months. In general, survival ofV. albo-atrum (47–69%) was higher thanV. dahliae (23 – 61%) following field burial of infected stems for 7 months.     

Plant-parasitic nematodes in the potato growing areas of Maine

A study was conducted from October 1987 to November 1988 to accumulate data on the distribution of plant-parasitic nematodes, to examine rotation schemes and time of sampling in relation to nematode populations, and to assess the occurrence ofVerticillium spp.-Pratylenchus spp. in potato growing areas of the state of Maine.Pratylenchus penetrans andP. crenatus were encountered most frequently, with detection being 19% higher in early season (May–June) than late season (September–November). All rotation schemes supportedPratylenchus spp.Meloidogyne hapla was encountered in 14% of the early samples and 19% of the late samples. Oats-potato, potato-potato, and clover-potato supported the highestM. hapla population levels. Other nematodes detected in fewer samples and in lower numbers than the two above genera wereParatylenchus spp.,Criconemella spp.,Helicotylenchus spp. andTylenchorhynchus sp. Two plants from each of 27 fields were examined for nematode-fungus associations. Samples from 7 fields were positive forV. dahliae andP. penetrans only,V. alboatrum andP. penetrans were found in 1 field,V. alboatrum alone was detected in 1 field andP. penetrans,V. dahliae andV. alboatrum were found together in 2 fields. The detection of these nematode-fungus associations suggests that the potato early dying complex defined elsewhere in the United States may be occurring in Maine.     

Long-Term survival ofCorynebacterium sepedonicum on contaminated surfaces and in infected potato stems

The survival of the ring rot bacterium,Corynebacterium sepedonicum, on contaminated surfaces and in infected stems was investigated by root inoculating potato stem cuttings with aqueous suspensions prepared from these materials. The pathogen survived for 24 mo on contaminated surfaces of burlap, kraft paper, and polyethylene plastic held at 12% relative humidity (RH) at either 5 or 20°C. It persisted for less than 14 mo on surfaces held at 94% RH at either temperature. Infectious ring rot bacteria were also recovered from dried, infected potato stems held for 26 mo in an unheated machine shed. These results emphasize the importance of strict sanitation and disinfestation procedures in maintaining potato seed stocks free of bacterial ring rot.     

Inoculum potential of Verticillium species in crop and weed residues overwintering in infested soil

Viable propagules ofVerticillium albo-atrum were recovered from roots and stems of naturally infected weeds and sunflower cultivars before or after burial in the field and from stems and roots of certain non-infested crops after burial in the field for seven months.Verticillium dahliae was recovered less frequently, then only from the roots ofSolanum nigrum (black nightshade). Survival of these pathogens after burial for 7 months varied considerably, ranging from 38 to 58% in sunflower stems and from 19 to 235% in roots of weeds. In stems of weeds and crops which did not contain propagules ofV. albo-atrum before burial, the fungus was recovered consistently after burial, indicating that saphophytic colonization probably occurred during interment.     

Testing for resistance to metalaxyl inPhytophthora infestans isolates from northwestern Washington

Outbreaks of late blight occurred in commercial potato fields in northwestern Washington in 1989 and reached epidemic proportions in 1990. Since most blighted fields reportedly had received 2–4 late-season applications of metalaxyl, the possibility existed that pathogenic strains ofPhytophthora infestans had developed resistance to this systemic fungicide. In testing this hypothesis,P. infestans was isolated consistently from diseased leaves, stems, and tubers of potato when small tissue pieces were surface-disinfected in 0.5% NaOCl for 3 min. Virulence of isolates was assessed on detached leaflets of eight differential (R-gene) potato genotypes. Although simple races (single and double) were recovered, there was also a high frequency of complex race combinations. When tested in metalaxyl-amended media, 81% of the 73 isolates recovered from diseased tissues were highly resistant to 10 μg metalaxyl/ml and 19% yielded intermediate resistance. Sensitive isolates were not recovered. Fifty-five percent of 40 isolates sporulated on potato tuber disks in the presence of ≥10 ug metalaxyl/ml. On leaf tissues, 85% of 20 isolates sporulated in the presence of 10 μg metalaxyl/ml, and 50% sporulated in the presence of 100 μg/ml. This is the first report ofP. infestans resistance to metalaxyl in the U.S.     

Root lesion nematodes in potatoes

Pratylenchus penetrans andP. crenatus were the dominant plantparasitic nematodes in potato roots and soil in Prince Edward Island. No nematodes were recovered from potato tubers. Forage legumes and grasses grown in rotation with potatoes also harbored large populations of root lesion nematodes. The amount of water moving through the stems of potato plants infected withP. penetrans in the greenhouse generally was less than in nematode-free plants;P. crenatus did not produce this effect. Telone-II, Telone-C-17, and Temik reduced the number of nematodes substantially in field plots. Yields of Superiors, Kennebecs, Sebagos and Netted Gems in nematicide-treated plots were not significantly greater than in untreated plots. There was a small but significant reduction in the specific gravity of tubers in the Telone-treated plots.     

Survival ofCorynebacterium sepedonicum in potato stems and on surfaces held at freezing and above-freezing temperatures

The survival ofCorynebacterium sepedonicum, the bacterial ring rot pathogen, in infected potato stems and on burlap surfaces held at various freezing and above-freezing temperatures was investigated by root-inoculating potato stem cuttings with aqueous suspensions prepared from these materials. Infectious bacteria were recovered from dried Russet Burbank stems held for 44 mo in an unheated machine shed and from dried Warba stems held for 55 mo. Inoculum from dried stems of all cultivars held for 63 mo did not incite symptoms; however, inoculum from Russet Burbank stems incited symptomless infection in 10% of the inoculated plants. This 63-mo survival period exceeds a 26-mo survival period previously reported. Ring rot bacteria survived and remained infectious for at least 18 mo on burlap surfaces subjected to temperatures of ?40° to ?5°C, alternating temperatures of -5° to 5°C, or a constant temperature of 5°C. Inoculum from burlap stored at the lower temperatures caused the most severe symptoms. These results stress the need for keeping crop debris away from potato operations and for using proper decontamination procedures to maintain potato seed stocks free of ring rot.     

Pratylenchus spp. andVerticillium spp. in New Brunswick potato fields

A survey of New Brunswick potato fields with symptoms of early dying was conducted in September 1990 and 1991. Root lesion nematodes (Pratylenchus spp.) were detected in 43 of 46 fields in 1990 and in 37 of 43 fields in 1991. In 1990 and 1991 populations averaged 910 and 410 nematodes g-¹ dry root and 1030 and 720 nematodes kg¹ dry soil, respectively. Previous crops did not affect nematode populations, except in one comparison where the number of nematodes in potato roots in 1990 was higher when the previous crop was cereal than in 1991 when the previous crop was peas.Pratylenchus crenatus was more prevalent thanP. penetrans. Root-knot nematodes(Meloidogyne hapla) were detected at a few sites at low population levels only in 1991. In 1991,Verticillium albo-atrum was detected in all 37 potato fields where fungal analyses were conducted, and the average population was 350 propagules g¹ dry soil. Soil populations ofV. alboatrum did not differ in fields in which either Russet Burbank or Shepody were being grown, nor did previous crops have any effect.Verticillium dahliae was detected in 17 fields at very low populations, usually below 1 propagule g¹ dry soil. There were no significant correlations (P<0.05) between populations of P.crenatus orP. penetrans in soil or roots and populations ofV. albo-atrum orV. dahliae propagules in soil.     

Recovery ofVerticillium dahliae from North American certified seed potatoes and characterization of strains by vegetative compatibility and aggressiveness

Samples of tubers collected from commercial potato seed lots produced across North America were assayed forVerticillium dahliae. V. dahliae was successfully isolated from 65 of 224 seed lots tested, a successful isolation rate of nearly 30%. Vegetative compatibility of the isolates was assessed through complementation tests using nitrate non-utilizing mutants. AllV. dahliae isolates belonged to vegetative compatibility group (VCG) 4. Of the 162 isolates recovered, 64% belonged to VCG 4A, 33% to VCG 4B, and 3% to VCG 4AB. All 39 of the isolates tested in the greenhouse on potato cv. Superior were pathogenic to potato. Disease symptoms developed earlier, were more severe, and plants died earlier when inoculated with VCG 4A compared to 4B isolates. As a group, AUSPC values were significantly higher (p=0.05) for VCG 4A than for 4B isolates. These data suggest that (1) commercial certified seed tubers from diverse locations are commonly infected withV. dahliae and thus may serve as primary sources of the pathogen; (2) potato isolates ofV. dahliae in North America belong to VCG 4A and 4B and these strains are widely distributed via seed tubers; and (3) VCG 4A and 4B are distinct pathotypes ofV. dahliae that vary in their aggressiveness to potato.     

Seasonal variation and effects of wheat rotation on populations ofVerticillium dahliae Kleb. in Ohio potato field soils

Soil population levels ofVerticillium dahliae in Ohio were monitored from May–October, 1982–1985, in 15 fields in potato-wheat rotation and two fields in potato monoculture. Population levels in fields in rotation ranged from 0 to 86 microsclerotia/10 g of air-dried soil with average values during each 6-month sampling period of 9.7, 12.9, 9.6 and 19.6 microsclerotia/10 g of soil for 1982, 1983, 1984 and 1985, respectively. In at least 2/3 of the fields sampled each year, soil populations ofV. dahliae peaked in either July or August, regardless of whether fields were under wheat rotation or potato monoculture. Of the 15 fields under potato-wheat rotation, 13 showed a general pattern of increased populations ofV. dahliae in one or both years following cropping to potato. Among those 13, significantly higher populations (P < 0.05) were observed in one wheat field in 1983 and in eight fields in 1985. In the two fields in potato monoculture, one consistently had population levels ofV. dahliae 3–4 times higher than any other field sampled in this study. Factors that may contribute to periodic changes inV. dahliae populations, implications of these changes in interpreting soil population data, and usefulness of a potato-wheat rotation in managingV. dahliae populations in Ohio are discussed.     

Fungal pathogens andPratylenchus penetrans associated with preceding crops of clovers,winter wheat,and annual ryegrass and their influence on succeeding potato crops on Prince Edward Island

The incidence of soilborne fungal pathogens of potatoes and root lesion nematodes were determined in preceding crops of clovers, annual ryegrass, and winter wheat, in the succeeding potato crops and soils and severity of associated potato diseases recorded in two, 2-yr field trials. Differences were not evident in yearly recovery ofRhizoctonia solani, Verticil-Hum dahliae, V. alboatrum, andColletotrichum coccodes from previous crops and soils. However, overall trends were present with a higher recovery ofV. dahliae from wheat and a greater recovery ofR. solani from clover than either wheat or ryegrass. A previous crop of ryegrass resulted in lower levels ofR. solani in succeeding potato soils in both trials and after clover in one trial at the pre-harvest sampling time. In trial 1 greater numbers ofPratylenchus penetrans were recovered from clover soils than from soils of ryegrass or wheat. One instance of lower nematode recovery in potato soils following ryegrass occurred but no differences in their recovery from potato roots were observed. Potato wilt incidence was lowest after clovers. Stem rot severity was positively correlated with recovery ofR. solani andV. dahliae from potato stems and roots and negatively correlated withV. alboatrum from potato stems only. Nematode populations were not related to potato disease symptom severity ratings. Results reported herein indicate that only a portion of soilborne diseases of potatoes on Prince Edward Island may be related to previous crops produced in the 3-yr rotation selected for this study.     

Effects of crop rotation and granular nematicides on the incidence ofVerticillium dahliae Kleb. andColletotrichum coccodes (Wallr.) Hughes,in potato

Summary The incidence ofVerticillium dahliae andColletotrichum coccodes was studied in a crop rotation experiment on sandy soil from 1983 to 1986. Early in the growing season the percentage of stems infected byV. dahliae increased with increasing cropping frequency of potato, depending on the cropping sequence, and decreased with the application of granular nematicides. However, later in the growing season the initial differences decreased.C. coccodes infections of stems were not affected by these factors. The percentage of plants infected byV. dahliae closely correlated with the percentage of plants with wilt symptoms but this relationship was absent forC. coccodes. Early in the growing season, the more vigorous the plants, the greater the level ofV. dahliae infection. Soil infestation withV. dahliae increased with increasing cropping frequency of potato but was not affected by annual applications of granular nematicides.     

The cause of deep-pitted scab of potatoes

By use of a tyrosine-casinate-nitrate medium,Streptomyces atroolivaceous, S. cinerochromogenes, S. corchorusii, S. diastatochromogenes, S. lydicus, S. malachiticus, and three unidentifiedStreptomyces spp. were isolated from deep-pitted scab lesions on potato cultivars Green Mountain, Red Pontiac, Russet Burbank, and Saco. Over 90% of the deep-pitted lesions contained one or more of theseStreptomyces spp., butS. scabies the cause of shallow scab was not isolated. Isolates ofStreptomyces spp. were also consistently isolated from aseptically excised non-necrotic tissue near the edge of lesions. TheseStreptomyces spp. appeared to be the primary colonizers of this tissue. Alternaria spp.,Fusarium spp.,Penicillium spp.,Rhizopus spp.,Trichoderma spp., and several unidentified non-sporulating fungi as well asBacillus spp.,Pseudomonas spp., andMicrococcus spp. were isolated from deep-pitted scab lesions. Fungal and bacterial isolates varied from one scab lesion to another and except for anAlternaria sp., were not pathogenic. Most of theStreptomyces spp. isolates used to infest heat pasteurized field soil caused deep and/or shallow scab on tubers of the potato cultivar Kennebec in greenhouse and outdoor plantings. Re-isolations from resulting scab lesions yieldedStreptomyces spp. similar in morphology to those used as inoculum. We concluded that a number ofStreptomyces spp. probably includingS. scabies (ATCC 3352 = S.griseus) may cause deep and shallow scab.     

The latex agglutination test as a rapid serological assay forCorynebacterium sepedonicum

The latex agglutination test was evaluated for detection of the potato bacterial ring rot pathogen,Corynebacterium sepedonicum. The bacterium was detected in infected potato stems and tubers tested and in inoculated eggplant stems. Test reliability was confirmed using blind tests. A minimum of 10⁶ bacterial cells/ml was requisite for observable agglutination. Nonspecific reactions were not observed from healthy extracts of plant tissues. Nonspecific reactions with other bacteria maintained in pure culture were eliminated by addition of bovine serum albumin to sensitized latex preparations.     

Antigenic anomaly in a naturally occurring nonfluidal strain ofCorynebacterium sepedonicum

A nonfluidal strain (SSNF50) ofCorynebacterium sepedonicum (=Clavibacter michiganense subsp.sepedonicum) was isolated from potato (Solanum tuberosum) stems from Idaho. A precipitin band common to other strains was not detected in Ouchterlony double diffusion (ODD) tests of SSNF50. SSNF50 was indistinguishable from fluidal strain SS20 ofC. sepedonicum with respect to latent period in eggplant or potato, reaction in latex agglutination and fluorescent antibody stain, and level of population in infected potato stems. Latent period of infection in potato from nonfluidal strain SSNF201, isolated in the laboratory from a culture of fluidal strain SS20, was longer than that of SS20 or SSNF50 despite similar pathogen population levels. A precipitin band common to ODD tests of SS20 and SSNF50 was not detected in tests of SSNF201. SSNF50 is the first nonfluidal strain ofC. sepedonicum to be isolated from potatoes.     

Status of weeds as reservoirs of plant parasitic nematodes in banana fields in Martinique

During a survey of the nematodes associated with weeds in banana fields in Martinique, 41 weed species in 37 genera from 20 plant families were collected to extract nematodes from the roots. Results of this survey showed that 24 weed species were hosts of Radopholus similis, 23 were hosts of Helicotylenchus spp., 13 were hosts of Pratylenchus spp., 13 were hosts of Hoplolaimus seinhorsti, 29 were hosts of Meloidogyne spp. and 24 were hosts of Rotylenchulus reniformis. The presence of the burrowing nematode was more consistently found within three families, the Euphorbiaceae, Poaceae and Solanaceae. In some weed species such as Caladium bicolor, Commelina diffusa, Echinochloa colona and Phenax sonneratii, the levels of nematodes recovered were similar to, or greater than the numbers recovered from Musa roots. These results clearly show that certain weeds can be significant reservoirs of plant parasitic nematodes including R. similis in banana fields. This information is crucial in devising appropriate nematode control measures for use with rotation crops or fallow before re-planting banana fields with nematode free planting material.     

Potato early dying disease in the Pacific Northwest caused byErwinia carotovora pv.carotovora andE. carotovora pv.atroseptica

Premature death of potato vines is widespread in irrigated potato fields of the Pacific Northwest.Verticillium dahliae is a major cause of early dying in fields with a history of potato production, but in fields new to potato production, or those that have been fumigated and have a lowV. dahliae population in the soil, premature death of vines can also occur.Erwinia carotovora pv.carotovora (Ecc) andE. carotovora pv.atroseptica (Eca) have been implicated in the early dying disease syndrome in these fields. Both bacterial pathogens are associated with symptoms similar to those caused byV. dahliae; i.e., a progressive chlorosis and necrosis of the foliage and vascular discoloration that may extend a short distance up the stem. Greenhouse pathogenicity studies on cv. Russet Burbank cuttings in which symptom development was identical to that produced byV. dahliae have verified thatEcc andEca can cause potato early dying.     

Factors affecting ring rot development in root-inoculated potato plants originating from stem cuttings

Some of the factors that affected ring rot development in potato plants grown from stem cuttings and root-inoculated withCoryncbacterium sepedonicum were investigated. Isolates ofC. sepedonicum cultured and stored for a, year on agar media were as virulent as those that had been cultured for almost 5 months. Isolates cultured for periods over 2 years gradually lost their virulence; however, decline in virulence was not as rapid as had been previously reported. One isolate became more virulent by inoculating and recovering it from a susceptible potato plant. Ring rot symptoms developed more rapidly in the cultivar Red Pontiac than in the cultivar Netted Gem; however, disease severity eventually reached the same level in both cultivars. The more mature the plants were when inoculated, the greater was the rate of ring rot development. Duration of exposure of wounded roots to inoculum of the pathogen did not affect disease development. No ring rot bacteria were recovered from plants originating from apical cuttings of ring-rot-infected potato plants that exhibited partial wilting. By this technique, plants infected withC. sepedonicum were freed from this pathogen quickly and effectively.     

Study on antagonistic effects of Talaromyces flavus on Verticillium albo-atrum, the causal agent of potato wilt disease

Talaromyces flavus a fungal antagonist, was isolated from soil samples collected from potato fields in Varamin and Karaj districts, Tehran province, Iran. Antagonistic effects of T. flavus isolates against Verticillium albo-atrum, the causal agent of potato wilt disease were investigated in the laboratory and greenhouse conditions. T. flavus colonies were recovered after three weeks from soil samples cultured on selective medium. Antagonistic effects of volatile and non-volatile extracts of T. flavus isolates on V. albo-atrum growth were investigated in the laboratory and five that caused higher growth inhibition of V. albo-atrum, were selected for greenhouse experiments. Infection index was compared in the greenhouse in a split plot trial with five isolates applied to soil, seed, or both arranged in a randomized complete block design with four replications. The minimum infection index was observed when seed were treated with T. flavus with the most effective isolate being Tf-Po-V-52. On seed, the minimum infection index was observed with Tf-Po-V-50. The most effective T. flavus isolate was also evaluated in a field experiment. Results indicated that infection index in seed treatment contained this isolate was 0.15 whereas that of control was 3.5. The overall results of this study showed that it may be possible to manage potato Verticillium wilt disease effectively by using T. flavus, a biocontrol fungus.     

The Effect of Alfalfa Residue Incorporation on Soil Bacterial Communities and the Quantity of <Emphasis Type="Italic">Verticillium dahliae</Emphasis> Microsclerotia in Potato Fields in the Columbia Basin of Washington State,USA

Verticillium wilt, caused by the soil-borne fungus Verticillium dahliae, is one of the most important diseases of potato in North America. Soil incorporation of alfalfa residues prior to planting potato could be a nonchemical Verticillium wilt management tactic by reducing the number of viable microsclerotia in field soil. Verticillium dahliae microsclerotia were quantified in field soils where organic material from alfalfa was incorporated, and numbers of microsclerotia were compared to fields where alfalfa residue was not incorporated. In addition, bacterial metagenomics was utilized to characterize soils where organic material from alfalfa was or was not incorporated to determine if alfalfa residue incorporation facilitates the formation of soils that suppress or kill V. dahliae microsclerotia. The number of V. dahliae microsclerotia in soil was greater (P = 0.0003) in fields where crop residue was incorporated than fields without incorporation when chloropicrin was used as a fumigant. Conversely, the number of V. dahliae microsclerotia observed in potato plants did not differ (P = 0.4020) between fields where residues were or were not incorporated if chloropicrin was used. Alfalfa residue incorporation did not significantly alter the soil bacterial metagenome compared to fields not subject to residue incorporation in both years of study. Despite these conclusions, the method can be employed to analyze the effect of grower practices with the intent of linking a field practice to increasing soil bacterial diversity and decreasing Verticillium wilt severity on potato.