Sunday, 17 February 2019

Nematodes Weed Control

Nematodes Weed Control

The use  of  nematodes  as  biological control agents  is  not a new concept nor  is  it restricted to the area  of  weed control. To the contrary, the majority of work and successes with nematode biological control agents has occurred in the area  of biological control  of  insects. Considerable energies have been expended with such diverse targets  as  the larch sawfly (Cephalicia lariciphila), carpenter worms (Prionoxystus robinae Peck), and Colorado potato beetle (Leptinotarsa decemlineata Say). Many  of  these systems employing the insect parasitic nematode Neoaplectana carpocapsae Weiser show promise as biological control options. The utilization  of  plant parasitic nematodes for the biological control  of  weeds  is  newer and less proven than the insect-nematode biological control strategies. All schemes involving the control  of  weeds by nematode agents center on the application of foliar gall-forming types.

Gall-forming nematodes have been known since 1743, when Needham discovered motionless eelwormlike organisms inside grains  of  wheat. Anguina tritici (Steinbuch) Chitwood, the casual agent for the galling of the wheat kernel, is  representative  of  the family Anguinidae, a grouping  of  the nematodes noted for their ability to induce malformations (galls) on foliage and floral portions of plants. The type genus Anguina almost exclusively infects monocots. Other genera within the family attack monocots and/or dicots.

Gall formation has been termed the growth reaction  of  the plant against the invasion  of  the parasite. In the case  of  gall-forming nematodes it appears that the nematode derives all the benefits from gall development, including shelter, nourishment, and perhaps even dispersal. Others have proposed that the development  of  galls may be a passive means  of  defense  by  the plant  in  an effort to shield itself from the full impact  of  the parasite. Gall formation has also been described as  a localization  of  the parasite that over time has forced the parasite into  an extreme form  of  specialization. Foliar galls formed by nematodes are characterized by a pronounced hypertrophy and cell proliferation  of  the mesophyll and by presence  of  a central cavity containing nematodes. Gall development may be related to increased auxin levels within infected plant tissues, and there  is speculation that the host-cell auxin level may be augmented by the nematode.

An important characteristic  of  foliar gall-forming nematodes  of  the Anguinidae is  their ability to survive dehydration.  ThiS  phenomenon  of  anhydrobiosis has been known since the time  of  Leeuwenhoek. A requirement for this state is a period  of  slow dehydration.  In  the induction phase  of  anhydrobiosis the nematodes coil into tight spirals. Glycerol induction occurs, and the formation  of polyalcohols like glycerol may contribute to the ability  of  the nematode to survive desiccation. The recovery phase is hypothesized  to  consist  of  a latent period (metabolism resumes) and a later recovery period during which movement  is resumed. Studies with the free-living mycophagous nematode  Aphelenchus avenae  indicate that lipid and glycogen declined rapidly upon dehydration, whereas glycerol and trehalose contents both increased rapidly. Strong correlations between survival  in  dry air and glycerol and trehalose contents were observed. The observed increase  in  glycerol may contribute to the survival  of  the nematode under dehydrated conditions by replacing the water structure around macromolecules and membrane systems. When rehydration occurs the morphological and metabolic changes that occur during the process  of  anhydrobiosis are reversed upon rehydration. A long period  of  slow dehydration  is  required for the nematodes to reenter another cycle  of  anhydrobiosis.

Folliar Gall-Forming Nematodes  of  Dicots

Goodey investigated the gall-forming nematode Subanguina millefoli (Low) Brzeski. The parasite has a wide plant host distribution, including several species of  Achillea. Observations  of  S. millefoli on millefolium  L.  indicated that twisting and distortion  of  infected plant parts did not adversely affect the "general health" of  the plant. Leaflets and stems were commonly infected by the nematode. Cells closest to the gall cavity contained granular protoplasm. Vascular tissues were more abundant than in healthy plant tissues. In studies of  A.  balsamophila (Thome) Filipjev infective nematode larvae entered young leaves  of  developing crowns  of  Balsamorhize sagittata. Infective larvae entered  in  mass at various points. At 3 weeks time, larvae matured into adults and began depositing eggs. During the normal growth cycle  of  B. sagittata the plants wither and die back in July or August. At this point the newly hatched larvae become quiescent. With the onset  of  spring the larvae escape the galled plant debris and congregate around developing crowns. Galled tissue  of  Wyethia amplexicaulis, another host  of  A. balsamophila, tends to exhibit considerable proliferation  of  leaf tissues, with the center  of  the gall being hollow. Cells with dense protoplasmic contents surround the hollow central chamber. The observations by Goodey with these two nematodes highlight the type  of  life cycle and development that  is  typical  of  foliar gall-forming nematode.

Biological  Control  Prospects for Anguinidae

Within North America several weed problems lend themselves to the potential use  of  plant parasitic nematodes  as  biological control agents. Where the nematode and weed have evolved naturally in North America,  an  augmentative biological approach  is  probably most appropriate, but  in  an introduced exotic weed, a classical biological control approach  is  feasible. Both methods have advantages and disadvantages. For an augmentative approach large numbers  of  biological control agents at a relatively low price are required. Introduced exotic agents require close examination to ensure they are free  of  pathogens and hyperparasites. Both approaches require attention to potential impacts  of  the biological agent upon plants  of  agronomic importance and native floras.

Common  Fiddleneck

Anguina amsinckia parasitize the common fiddleneck, Amsinckia intermedia, an annual, native weed in California. This weed poses a potential problem to grazing livestock, since, if ingested in sufficient quantities, it can be toxic. Infection  of  plants by the nematode tends  to  be geographically localized. Actualgalling  of  plant tissues  is  often limited to the fruits and leaves. Galls are characterized by a central cavity surrounded by dead empty cells. More galling may occur within fruit and the apical meristems where the nematode  is  protected from desiccation.

Higher humidity levels provided  by  the stem apex appear  to  change when the inflorescence elongates. Pantone et al. consider this a factor that curtails nematode movement and subsequent infection. The exposure period during which floral parts are susceptible  to  nematode infection  is  short and may limit the utility  of this organism  as  a biological control agent. Other factors, such  as  host range, may also influence use  of  this agent.

Silverleaf Nightshade

Silverleaf nightshade,  Solanum elaeagnifolium  Cav., a native  to  North America,  is  an economically important perennial weed throughout much of the southwestern United States. It has also become a significant pestiferous weed in  several countries.

In Australia the weed  is  considered a serious threat  to  both crops and pastures. The first infestation  in  Australia was reported  in  1901. Control has been difficult because  of  the ability  of  the weed  to  recover following cultivation.  S.  elaeagnifolium  is  especially annoying in the states  of  south Australia and Victoria, where it competes with pasture grasses, reducing yields.

The entry  of  S.  elaeagnifolium  into India  is  conjectured  as  being via a contaminant  of  imported food grains. The foliar gall-forming nematode  Orrinaphyllobia  (native to North America) also gained entrance  to  India possibly through infected, dried plant debris.

South Africa also recognizes  S.  elaeagnifolium  as  a serious weed pest. Herbicides have not been successful in controlling the weed, and mechanical control is  not practical because of the deep-spreading root system that easily regenerates new plants upon being damaged. In 1973, Neser viewed  S.  elaeagnifolium  as  a prime target for biological control.

In the life cycle  of  o.  phyllobia.  adults and preinfective juveniles develop only within the moist microhabitat  of  foliar and stem galls. When galled leaf and plant material abscises and dries, the adults and pre infective juveniles die. The infective stage larvae are able to enter a state  of  anhydrobiosis in which they can remain viable for several years. This  is  especially useful  in  a biological control program, since the inactive form can be applied at a time convenient for the grower. The nematode will revive when conditions are suitable for its development. The dormant stage  is  able to revive within several hours following rehydration of the gall material. The natural infection cycle commences during periods  of  extended moisture when the larvae rehydrate, become motile, and exit the galls. In this free state within the upper soil surface, the infective larvae can infest preemergent or emerged shoots. In their dried anhydrobiotic state nematodes are dispersed by wind-blown gall and other infected plant debris. Orr determined that large numbers  of  infective larvae can be readily introduced into  S.  elaeagnifolium populations by merely spreading dried gall material within the weed stand. Once introduced, the nematodes spread rapidly and significantly reduce top growth  of the weed.

Typical gall development  is  detectable  0-1.5  days following the initial infection period. Larvae can be observed moving among the plant trichomes and become concentrated among apical leaf folds. This concentration  of  nematodes at the apical leaf folds continues, with actual penetration  of  the leaf epidermis occurring sometime during the first day. Larvae  of  O.  phyllobia  are often found lodged in wounded areas  of  stems. The nematode appears  to  be attracted toward members of  the Solanaceae, particularly  Solanum  species. Larvae are not selective  of  which stems they ascend, virtually ascending any available stem that exhibits proper moisture conditions. Penetration usually occurs through the adaxial surface.
At  2-4  days, minute swellings less than 2 mm in diameter appear along leaf veins, midribs, petioles, axillary buds, and stems. Within infected tissues, nematodes tend to localize in the palisade and cortical parenchyma.  By  the third day of  development the infected areas contain larger intercellular spaces and the number  of  nematodes observed increases. At this point nematodes can be observed molting, and distortion and swelling of palisade layer commences. Minute swellings along veins and petioles enlarge into small galls less than 5 mm in diameter. Occasionally, nematodes advance into mid-portions  of  the leaf. At  3-4  days, gravid females and small numbers  of  eggs can be found within the developing galls. Seven to 9 days following initial infection, galls consist of loosely packed tissue composed primarily  of  intercellular spaces with all developmental stages of  the nematode present. Larvae begin to migrate into previously uninfected areas. Highly developed galls at volumes up to 5 cm 3  are evident at  15  days. By 30 days, second-generation females mature and begin active egg deposition. At this time actual nematode numbers within individual galls may exceed several hundred thousand.

The potential  of  utilizing O.  phyllobia,  an  endemic North American species, as  a biological control agent was first contemplated  by  Notham and Orr. Orrina phyllobia  is  commonly found in roadside areas and abandoned fields throughout most  of  Texas and northeast Mexico. Preliminary host range studies indicated that O.  phyllobia  was host specific for  S.  elaeagnifolium.  Several crops, including  Lycopersicon esculentum  Mill.,  S.  tuberosum  L.,  and  Gossypium hirsutum  L.,  were tested in the original plant screening. All proved negative for infection by O.  phyllobia.  Later studies indicates that  S.  melongena  L.  did serve as a limited host plant. Recent work in South Africa indicates that the host range  of  O.  phyllobia  may be more extensive then previously known. Neser et al. expressed doubts about widespread utilization of  O.  phyllobia  due in part to concern in South Africa over the potential impact  of  the nematode upon endemic Solanaceae species. Screening  of  endemic natives showed galling on  S. coccineum  Jacq., S.  Burchelli  Dun., and  S.  panduriforme  E. Mey. although galling on the South African endemic species  was  less extensive than that experienced with  S.  elaeagnifolium.

Mass Production of  Orrina Phyllobia

An economical mass production scheme  is  required to utilize O.  phyllobia  in  an augmentation biological control program against  S.  elaeagnifolium  within the United States. At the U.S. Department  of  Agriculture, Animal and Plant Health Inspection Service Mission Biological Control Laboratory, Mission, Texas, a field production scheme was developed and employed in the mass production  of O.  phyllobia.  The field production scheme requires an adequate dense stand  of S.  elaeagnifolium  for optimum production. Most natural stands  of  the weed are too restricted in size or too scattered to  be  of  use  in  mass production. Also the availability  of  irrigation lessens dependency upon rainfall for production purposes and allows multiple cropping during the growing season.

Several different methods were employed in the establishment of the  S.  elaeagnifolium.  The seed was planted directly into rows or broadcast onto the prepared field. Germination by both  of  these methods with and without irrigation was nil. Successful germination was achieved by germinating seed in the greenhouse in seed fiats and transplanting  to  peat pots. Plants were grown from  20  to  30 cm in height prior to transplanting to the field. Seedlings were transplanted 30 cm apart with 76-cm row spacing. Field production plots were irrigated and weeded. Plants grew well under these conditions, with the plant canopy covering the entire plot after 4 months. Total area planted by this method has exceeded 1 ha. Prior  to  the inoculation  of  field plots, mature S.  elaeagnifolium  plants were disked under the soil surface. Plots were inoculated by broadcasting nematode inoculum (dried galled leaves containing 30,000+ nematodes per gram) onto the plot at a rate  of 3.4-11.2  kg/ha. Light disking  of  plots helped  to  incorporate inoculum into the upper soil surface. A modified sprinkler system was employed to maintain moist conditions for a minimum  of  16-20 hours following inoculation. Galls resulting from inoculation were evident several weeks following initial infection.

Harvesting  of  gall material commenced  4-5  weeks following the infection period, depending upon local weather conditions. Moist climatic conditions speeded up gall formation, whereas drier, hotter conditions tended  to  slow gall development. Galled plants were individually collected so  as  not  to  dilute the nematode inoculum with un infected plant tissue.

Collected infected galls were dried under forced air at a temperature of 37°C for  24-36  hours. Drying time was dependent on air humidity levels. The dryer employed at the Mission Laboratory  was  a modified tobacco dryer design. Gall material was placed on screened trays, which were placed  in  racks with forced heated air circulating through the unit. Following drying, stems and berries were stripped off and leaf gall inoculum was double bagged  in  polyethylene bags. All inoculum was then stored at  -17°C.

Unpublished estimates  of  the cost  of  this production system ranges from $55-60  per pound  of  dried galled leaf material. This price was not considered competitive with currently available weed control strategies. Other techniques  of mass-producing the nematode may offer opportunity  to  reduce costs associated with inoculum production and make the biological control option more competitive.

Russian Knapweed

Acroptilon rep ens  L.  DC. (Russian knapweed)  is  a persistent, introduced perennial weed  in  North America. The combination  of  an extensive root system coupled with the plant's allelopathic properties may be partially responsible for its strong competitive ability.  In  dense infestations  it  inhabits other plant growth and reduces yields  of  important agronomic plants. Besides its strong competitive growth,  A. repens  is  also considered a toxic weed, being especially poisonous to horses. Probable entry into North America occurred about 1900 with importations  of Turkestan alfalfa.  In  North America A. repens  is  relatively free  of  specialized parasites and  is  not extensively attacked  by  polyphagous feeders. A. repens propagates primarily by vegetative means. The inability  of  A.  repens to set seed has lessened the potential effectiveness  of  introducing various seed head biological control agents, such  as  Aceria acroptiloni, Urophora maura, and Dasyneura spp. Emphasis has been directed toward biological control agents that attack vegetative portions  of  the plants. The nematode Subanguina picridis (family: Anguinideae) causes extensive damage  toA.  repens. A native  to  southern portions  of  the  U.S.S.R.,  it has a worldwide distribution.

Extensive work has been done by A.  K.  Watson in Canada on the release, establishment, and biology  of  Subanguina picridis, a foliar gall-forming nematode  of  Acroptilon repens  L.  The organism  is  currently used within the U.S.S.R. as a biological control agent  of  Russian knapweed.  Host races may have developed, although the possibility  of  up  to  six different species  of  the genus has been dismissed. All "races"  of  the genus are known to attack members of  the Cynareae tribe. Morphological differences between various populations exist, depending upon which host plant the nematode  is  retrieved from, but these differences are not considered distinct enough to justify separate species status.

Recent work by Watson indicates that the actual host range  of  S.  picridis is  more extensive than originally speculated. Gall formation was noted on various members  of  Centaureinae, including  A.  repens, Centurea diffusa,  C.  maculosa, Carduus nutans, Cirsium fiodmanii, Cynara scolymus, and Onapordum acanthium. Representatives from two other families, Echinopinae and Mutisieae, have also exhibited galling infection by  S.  picridis. For the purposes  of  Watson's studies, host plant response was divided into two categories,  
(1)  resistant plants and 
(2) susceptible plants.
Resistant host plants exhibited good growth with poor parasite reproduction, and susceptible plants showed poor growth and good parasite reproduction. Watson considered only one  of  the screened species to be susceptible, C.  maculosa.  Of  economically important crops screened,  Carthamus tinctorius  (safflower), a close relative to  A.  repens,  was resistant with no gall formation.  S.  picridis  did induce gall formation and was able  to  reproduce on C. scolymus  (Globe artichoke), although the infection level was low.

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