J. Katan

Interactions between herbicides and plant pathogens

The increasing use of herbicides for the control of undesired vegetation has in recent years had a tremendous impact on crop production as well as on nonagricultural land. It seems that the use of herbicides will continue to expand in the future. Selective herbicides for agricultural use are screened on the basis of their selective phytotoxicity, namely, their high toxicity to weeds and lack of damage to agricultural crops. The biological activity of any pesticide1 is usually not restricted to the target organism but extends to nontarget organisms as well. Hence, inhibitory and stimulatory effects on beneficial or harmful nontarget organisms in the environment are possible. It seems, however, that far less emphasis has been placed on the indirect effects of herbicides than on the direct ones, probably due to the complexity of the problem. Herbicides may affect crop plants in addition to target weeds either directly by causing phytotoxicity or indirectly by their effects on other organisms which may lead to harmful or beneficial effects. Crop plants form various kinds of relationships with other organisms, e.g., pathogens2 (agencies which incite disease) (Walker 1969). Plant disease is the final result of a compatible interaction which occurs under suitable conditions between a particular pathogen and host. A third living component connected with disease is the surrounding microflora and fauna which may affect either the pathogen, the host, or both by their antagonistic or synergistic action. Herbicides in their original form or as degradation products may interact in different ways with any one of the organisms involved in the disease, at one or more points in the chain of events leading to disease development (pathogenesis). The final result may be an increase, a decrease, or no change in disease severity or its incidence. One extreme case would be a severe outbreak of a “new” disease which was of negligible importance before the application of the herbicide, whereas the other extreme would be the complete elimination of an existing severe disease. The effect on disease might be immediate during the same growing season, or it might be a long-term effect due to build-up, especially where persistent herbicides are involved. Changes in disease incidence were also observed following the use of insecticides and fungicides (Munnecke 1967).

Effect of solar heating of soils by transparent polyethylene mulching on their chemical properties.

Solar heating of soils by mulching with transparent polyethylene during the hot season results in increased soil temperatures and the killing of certain pathogens. Mulching increases temperatures by up to 10 to 12°C in the upper soil layer. We studied the effect of this treatment on the chemical properties and hydraulic conductivity (HC) of soil. Field experiments showed enhanced plant growth in solar heated soils, even in the absence of known pathogens. Saturated extracts of the upper soil layers of eight different solar heated soils showed increased concentrations of soluble organic matter and minerals. The greatest increase was in NO3- concentration; NH4+, K+, Ca2+ + Mg2+, and Cl- were also found to increase. Changes in soil pH, total organic matter, NaHCCv extractable P, and HC were small or inconsistent. Tomato seedlings grown on extracts of heated soils showed enhanced growth in comparison with seedlings grown on extracts of unheated soils.

Vegetative compatibility groups of Verticillium dahliae in Israel : Their distribution and association with pathogenicity

A collection of 565 isolates of Verticillium dahliae, recovered between 1992 and 1997 from 13 host plant species and soil at 47 sites in Israel, was tested for vegetative compatibility using nitrate-nonutilizing (nit) mutants. Three vegetative compatibility groups (VCGs) were found and identified as VCG2A (28 isolates), VCG2B (158 isolates), and VCG4B (378 isolates) by using international reference strains. One isolate was heterokaryon self-incompatible. Of the VCG2B isolates, 92% were recovered from the northern part of Israel and 90% of VCG4B isolates were recovered from the south, with some overlap in the central region. Isolates of the minor group VCG2A were geographically scattered among the two major VCGs. Isolates of the same VCG resembled one another more than isolates from different VCGs based on colony and microsclerotial morphology, temperature responses, and, partially, pathogenicity. Different pathotypes were defined among 60 isolates tested, using cotton (cv. Acala SJ-2) and eggplant (cv. Black Beauty) as differentials. All isolates in VCG2A and 86% of the isolates in VCG4B, irrespective of their origin, induced weak to moderate symptoms on cotton and moderate to severe symptoms on eggplant and were similar to the previously described cotton nondefoliating patho-type. In contrast, all cotton isolates in VCG2B caused severe foliar symptoms, stunting, and often death, but little or no defoliation of inoculated cotton plants. These were defined as a cotton defoliating-like pathotype and induced only weak to moderate symptoms on eggplant. We concluded that vegetative compatibility grouping of V. dahliae in Israel is closely associated with specific pathogenicity and other phenotypic traits.

An RFLP marker in tomato linked to the Fusarium oxysporum resistance gene I2

SummaryThe locus, I2, which in tomato confers resistance against Fusarium oxysporum f. sp. lycopersici race 2, was introgressed into Lycopersicon esculentum from the wild species L. pimpinellifolium (P.I. 126915). We searched for restriction fragment length polymorphisms (RFLPs) between nearly isogenic lines (NILs) in clones that map to the region introgressed from the wild species. Since I2 maps to chromosome 11, we used DNA clones from this chromosome as hybridization probes to Southern blots containing bound DNA of the NILs digested with 23 restriction enzymes. Of the 14 chromosome 11 clones, 9 exhibited polymorphism. These clones were further hybridized to “verification” filters that contained DNA from resistant and susceptible L. esculentum varieties digested with the enzymes that gave the polymorphism. One clone, TG105, was found to be associated with I2; 19 susceptible lines showed a different RFLP with this probe than 16 resistant lines, including the original L. pimpinellifolium accession used as a source for the resistance gene. These results together with our mapping analysis indicate that TG105 is closely linked to the resistance gene.

Toward Integrated Management of Monosporascus Wilt of Melons in Israel

The phenomenon of melon wilting dueto Monosporascus infection (Fig. 1) isknown in many regions around the worldand has been referred to as melon collapse(13,27,32,38), sudden wilt (2,6,8,26,29),root rot (17,39), vine decline (1,3,36), androot rot and vine decline (21–23,41,42).This disease is known also in the ’AravaRift Valley of southern Israel (8,19,26,31;Fig. 2), and the major causal agent isMonosporascus cannonballus Pollack &Uecker (Fig. 3). This pathogen is commonin hot, semiarid melon-growing areas ofIndia (22), southern Spain (13), southwest-ern regions of the United States (23,24,36),Saudi Arabia (15), Central America (1),Japan (40), Taiwan (37), and Tunisia (21).This disease in the ’Arava can be verysevere, capable of destroying the entirecrop (26), and will be referred to here asMonosporascus wilt. To date, disease man-agement in the ’Arava (38) has beenmainly based on methyl bromide fumiga-tion of the soil prior to planting. Sincemethyl bromide use will be prohibited inthe near future (33), there is an urgent needto develop alternative strategies for diseasemanagement.Melon root rot and vine decline causedby M. cannonballus has been reviewed in afeature article by Martyn and Miller (22),which describes the biology, pathology,and epidemiology of the disease, as well asmolecular methods for detecting variationin the pathogen population. In this article,we discuss approaches for the control ofMonosporascus wilt, with an emphasis onthe potential for integrated management, inview of the coming phaseout of methylbromide. These approaches include breed-ing for resistance, grafting melon plantsonto resistant Cucurbita and melon root-stocks, changes in irrigation schemes, im-proved soil solarization, chemical controlwith fungicides, and the use of other fumi-gants, alone or combined with soil solari-zation, to improve disease control.In field trials conducted by Reuveni andKrikun in the Jordan Valley and southern’Arava region in Israel (Fig. 2) in the early1980s, it was shown that Monosporascuseutypoides (apparently synonymous withM. cannonballus) is the primary agent ofmelon collapse (19,31). Pathogenicity testsperformed in 1995 and 1996 suggested thatthe most virulent species involved in themelon collapse syndrome in the ’Arava(Fig. 2) is M. cannonballus, although otherpathogens might also be involved (26).Monosporascus appears to be adapted tohot climates. This can be inferred from theclimatic conditions in the areas in whichthe fungus has been found and by itsgrowth temperature optimum. Vegetativemycelial growth is extensive in the rangeof 25 to 35°C, and perithecia formation invitro is optimal at 25 to 30°C (22).In commercial fields in Israel, the meloncrop can be totally destroyed by Monospo-rascus wilt in the autumn cropping season,whereas disease incidence and severity in acrop raised in the same plot during thefollowing winter–spring season can bemuch lower (2). Differences in soil tem-perature between crop seasons have beensuggested as a possible cause for such aphenomenon (17,28). This idea has beensupported by enhanced wilting obtainedfollowing artificial heating of the soil dur-ing the winter–spring crop season (28).Soil fumigation with methyl bromidebefore planting is the most common ap-proach for controlling Monosporascus wiltof melons in Israel. Methyl bromide haslethal exposure periods as short as 2 daysand can be applied at relatively low tem-peratures. The aeration period to eliminatevolatile residues before planting is short inmost soils, 3 to 10 days, allowing plantingshortly after treatment (18). In fact, meloncultivation in the ’Arava region is ex-tremely risky without methyl bromidefumigation prior to planting, due to theubiquity of M. cannonballus in ’Aravasoils that results in severe yield losses. Thephaseout of methyl bromide in developed

Biological control of Sclerotium rolfsii and Verticillium dahliae by Talaromyces flavus is mediated by different mechanisms.

ABSTRACT Ten wild-type strains and two benomyl-resistant mutants of Talaromyces flavus were examined for their ability to secrete the cell wall-degrading enzymes chitinase, beta-1,3-glucanase, and cellulase, to parasitize sclerotia of Sclerotium rolfsii, to reduce bean stem rot caused by S. rolfsii, and to secrete antifungal substance(s) active against Verticillium dahliae. The benomyl-resistant mutant Ben(R)TF1-R6 overproduced extracellular enzymes and exhibited enhanced antagonistic activity against S. rolfsii and V. dahliae compared to the wild-type strains and other mu tants. Correlation analyses between the extracellular enzymatic activities of different isolates of T. flavus and their ability to antagonize S. rolfsii indicated that mycoparasitism by T. flavus and biological control of S rolfsii were related to the chitinase activity of T. flavus. On the other hand, production of antifungal compounds and glucose-oxidase activity may play a role in antagonism of V. dahliae by retardation of germination and hyphal growth and melanization of newly formed microsclerotia.

RFLP mapping of I1, a new locus in tomato conferring resistance against Fusarium oxysporum f. sp. lycopersici race 1

SummaryThe inheritance and linkage relationships of a gene for resistance to Fusarium oxysporum f. sp. lycopersici race 1 were analyzed. An interspecific hybrid between a resistant Lycopersicon pennellii and a susceptible L. esculentum was backcrossed to L. esculentum. The genotype of each backcross-1 (BC1) plant with respect to its Fusarium response was determined by means of backcross-2 progeny tests. Resistance was controlled by a single dominant gene, I1, which was not allelic to I, the traditional gene for resistance against the same fungal pathogen that was derived from L. pimpinellifolium. Linkage analysis of 154 molecular markers that segregated in the BC1 population placed I1 between the RFLP markers TG20 and TG128 on chromosome 7. The flanking markers were used to verify the assignment of the I1 genotype in the segregating population. The results are discussed with reference to the possibility of cloning Fusarium resistance genes in tomato.

Plant waste-based composts suppressive to diseases caused by pathogenic Fusarium oxysporum

The suppressive ability of three plant residue-based composts that could serve as components of soilless media for several vegetable crops was tested on four different formae speciales of Fusarium oxysporum: melonis, basilici, radicis-lycopersici and radicis-cucumerinum. The composts were prepared under controlled conditions from a mixture of separated cow manure (SCM) with orange peels (OP), wheat straw (WS), or dried tomato plants that had been removed from the greenhouse after the end of the season (TP). Disease development in melon, tomato and cucumber seedlings growing in the three composts was significantly less than that observed in peat. Plant inoculation was achieved by conidia produced in culture, conidia naturally produced on infected stems and soil inoculum produced by enriching the soil with infected tissues. Pathogen colonization of the roots and stems of infected melon plants grown in TP–SCM and OP–SCM composts was significantly lower than that of peat-grown plants. Sterilization by gamma irradiation reduced the suppressive capability of TP–SCM and OP–SCM composts, whereas it did not affect the disease development and final disease incidence in peat. Tested formae speciales exhibited differing decline rates of the conidia incorporated in the composts, compared with the rate in the peat control, which suggests that different mechanisms may be involved in the suppression of the different pathogens. The present study shows that composts based on plant-waste residues suppress diseases caused by different formae speciales of Fusariumoxysporum.

Physical and cultural methods for the management of soil-borne pathogens.

Abstract There is an increasing interest in using physical methods and cultural practices (CP) in disease control as alternatives to pesticides for the management of soil-borne pathogens. These can be used alone or as components of pest management programs. In this regard, there are three categories of CP: (a) CP for regular purposes which can also be used for disease control, e.g. irrigation; (b) CP which are used solely or mainly for pest control, e.g. sanitation; (c) CP which can be used for both agricultural purposes and pest control, e.g. crop rotation. The basic principles of CP for pest control are (a) any potential control method may be considered, providing that it is environmentally, technologically and economically feasible; (b) pesticide usage is minimized by combining with other non-chemical or chemical methods; (c) diseases that are difficult to control or that involve problematic pesticides, e.g. methyl bromide, should be prioritized; (d) economic aspects are taken into consideration. Physical methods include heating the soil or propagation material, irradiation, etc. CP for pest control can be used before, at or after planting. They include crop rotation, fallow, flooding, deep ploughing, flaming, soil solarization — which involves a combination of physical and biological processes, adjusting planting date, irrigation, fertilization, compost, weed control, herbicide application, sanitation, tillage and others.

Comparative Study of Genetic Diversity and Pathogenicity Among Populations of Verticillium Dahliae from Cotton in Spain and Israel

Genetic diversity and phenotypic diversity in Verticillium dahliae populations on cotton were studied among 62 isolates from Spain and 49 isolates from Israel, using vegetative compatibility grouping (VCG), virulence and molecular assays. In Spain, defoliating V. dahliae isolates (D pathotype) belong to VCG1, and non-defoliating isolates (ND) belong to VCG2A (often associated with tomato) and VCG4B (often associated with potato). The D pathotype was not identified in Israel. The ND pathotype in Israel is comprised of VCG2B and VCG4B. Isolates in VCG2B and VCG4B ranged in virulence from weakly virulent to highly virulent. The highly virulent isolates induced either partial defoliation or no defoliation. Virulence characteristics varied with inoculation method and cotton cultivar. Highly virulent isolates from Israel were as virulent as D isolates from Spain under conditions conducive to severe disease. The D pathotype is pathologically and genetically homogeneous, whereas the ND pathotype is heterogeneous with respect to virulence, VCG, and molecular markers based on single-primer RAPD and on PCR primer pairs.