Spain is one of the major producing countries of artichoke ( Cynara cardunculus var. scolymus ). In 2016, 16,001 ha were cultivated, with a total yield of 225,619 t. The country is also one of the largest exporters of flowers for fresh consumption or for use in conserves. Some 58.7% of the crops are in the Southeast of the country (provinces of Alicante and Murcia with 2,135 ha and 7,259 ha, respectively), with a production of 135.214 t ( MAPAMA, 2017 ). In southeast Spain, the cropping is biannual or annual, depending on the incidence of viruses such as Tomato spotted wilt virus (TSWV) and crop yield ( Lacasa et al. , 1996 ). It is planted in late August and early September; the harvest finishes between late April and mid-May. Most of the surface area is planted with vernalized stumps from other production zones ( Lacasa et al. , 2016 ). There has been an increase in the surface area dedicated to growing seed plants (principally, hybrid cultivars) in recent years. The soils are clay loam, with a low nitrogen and organic matter (OM) content, average electrical conductivity and a pH of 7 to 8. The climate in the southeast of Spain is Mediterranean, with a maritime influence on the coast and a continental influence inland. The summers are hot, while the winters are mild; precipitations are scarce and are concentrated in autumn and winter. Artichoke crop is rotated with other horticultural crops such as broccoli, melon, lettuce or potato, with artichoke being grown in the soil again every two or three years. The crop intensity or crop rotation with others crops which are susceptible to the same soil pathogens ( Ortega & Pérez, 2007 ), causes that such pathogens become a limiting factor of crop yield and emergence of phenomena of soil fatigue caused by the soil microbiological component. These phenomena reduce the soil fertility and crop yields in the same way that occurs in protected pepper crops grown in southeastern Spain ( Martínez et al. , 2009 ; Guerrero et al. , 2014 ).

In Levante and the southeast of Spain, Verticillium dahliae and Rhizoctonia solani are the principal soil-borne pathogens, causing important production losses ( Cebolla et al. , 2004 ; Armengol et al. , 2005 ). In the absence of a host plant, the most important pathogen in artichoke, V. dahliae, may persist as melanized microsclerotia in the soil for more than 10 years ( Cirulli et al. , 1994 ; Pegg & Brady, 2002 ). Both of these pathogens are frequent and abundant in soils in Alicante and Murcia where artichoke is grown ( Ortega & Pérez, 2007 ), and also in other crops such as melon or potato. Prospections carried out in the Region of Murcia between 2014 and 2017 found differences in the crops prevalence of both pathogens, depending on the production zones ( Guerrero et al. , 2017 ). This prevalence appears to be related with the farming practices used, in particular with the type of irrigation, and with the environmental conditions.

The strategies to control both diseases are based on planting in uncontaminated soils, the use of stumps which are pathogen-free or the use of seed plants, in crop rotation programs with crops that are not susceptible to the two pathogens ( Cirulli et al. , 1994 ). Soil chemical disinfestation in pre-planting using general disinfes­tants such as methyl isothiocyanate generators (metam so­dium, metam potassium or dazomet) was satisfacto­rily indicated in Italy ( Cicarese et al. , 1985 ), although it showed deficiencies in Levante, Spain ( Cebolla et al. , 2003 ). The combination of metam sodium and broccoli residues did not improve V. dahliae control when the broccoli residues were used alone ( Berbegal et al. , 2007a ) nor with ovine manure ( Cebolla et al. , 2003 ). The limitations in the use of methyl isothiocyanate general disinfectants (one application each three years in the same soil, Directive 2011/53/EU and Regulation (EU) n° 359/2012) condition the control strategy of both pathogens today. Pre-planting disinfestation with chloropicrin or with the mixture of chloropicrin and 1,3-dichloropropene is not authorised in Spain, although it has been shown to be effective ( Cebolla et al. , 2003 ).

The approach of combining solarization and OM application is defined as biological solarization or biosolarization ( Katan, 2005 ; Ros M et al. , 2008 ; Domínguez et al. , 2014 ). The effects of biosolariza­tion on different soil-borne pathogens are associated with the action of several mechanism such as i) the solarization temperature ( Katan et al. , 1976 ; Katan, 1981 ); ii) the effect of released gases from organic amendment biodecomposition ( Kirkegaard et al. , 1993 ; Kirkegaard, 2014 ; Butler et al. , 2011 , 2012 ); iii) the effect of anaerobiosis due to oxygen deficit in the tarped soil, mechanism that, according to the different countries, is denominated by several terms such as, biological soil disinfestation ( Blok et al. , 2000 ; Goud et al. , 2004 ; Momma et al. , 2006 ; Messiha et al. , 2007 ), anaerobic soil disinfestation ( Butler et al. , 2012 ; Rosskopf et al. , 2014 ; Serrano et al. , 2017 ), soil reductive sterilization ( Yossen et al. , 2008 ) or reductive soil disinfestation ( Katase et al. , 2009 ); and (iv) the amendment suppressive effect favouring microorganisms development which exert antagonistic effects on the pathogens ( Gamliel & Stapleton, 1993 ; Mazzola et al. , 2001 , 2007 ; Bonanomi et al. , 2007 ). The MBTOC (2007) denominated all these ways of disinfestation as biodisinfestation, since several actions concur when applying OM in any of these soil disinfestation techniques.

In the countries of the Mediterranean basin, sola­rization is presented as an effective disease manage­ment strategy for the control of V. dahliae and other soil-borne pathogens in numerous horticultural crops ( Katan et al. , 1976 , Katan, 1981 ; Cenis & Fusch, 1988 ; González et al. , 1993 ; Tamietti & Valentino, 2001 ), in orchard crops ( López-Escudero et al. , 2003 ; Yolageldi et al. , 2012 ) and cotton ( Melero et al. , 1995 ). Solarization has only been proven as a successful control strategy against Sclerotinia sp. and R. solani in Sicily and Tuscany ( Triolo et al. , 1985 , 1989 ; Cartia, 1987 ; Tamietti & Garibaldi, 1989 ). Solarization was effective in controlling Verticillium wilt of artichokes for three successive cropping seasons in Greece ( Tjamos & Paplomatas, 1988 ). The incorporation of OM into the soil prior to the solarization (biosolarization/biodisinfestation) improves the efficacy of pathogen control in horticultural crops in greenhouse or in open field ( Gamliel & Stapleton, 1993 ; Bonanomi et al. , 2007 ; Guerrero et al. , 2010 ; Núñez- Zofio et al. , 2011 ; Domínguez et al. , 2014 ). In Levante, Spain, solarization of cauliflower residues mixed with ovine manure was shown to be effective for Verticillium control in artichoke crops ( Cebolla et al. , 2003 , 2004 ; Berbegal et al. , 2007a , 2008 ) through soil inoculum reduction and significantly diminished the percentage of plants displaying symptoms and which were infested, and increased crop yield.

In protected pepper crops located in southeastern Spain, biosolarization improves the efficiency of sola­rization in the control of soil-borne pathogens such as Meloidogyne incognita ( Ros C et al. , 2008 ) and reduces the effect of soil fatigue in the absence of the main pathogens ( Phytophothora capsici, P. parasitica and M. incognita ) which resulted in a crop yield increase ( Martínez et al. , 2009 ; Guerrero et al. , 2014 ).

Given the difficulties in using chemical disinfes­tants to reduce the inoculum of Verticillium and Rhizoctonia in soil dedicated to growing artichoke and the possibility of using new local organic amend­ments, there is a need to evaluate the efficacy of new organic amendments for the control of R. solani and V. dahliae , the main soil-borne pathogens of articho­ke crops in southeastern Spain. The aim of this work was to assess the effects of biosolarization using organic residues from local agri-food industries as organic amendments for the control of V. dahliae and R. solani in artichoke crops.

The inoculum density and the incidence of Verti­cillium wilt in the artichoke crop were significantly reduced by biosolarization and, an increase in marketable yield was achieved for two consecutive years. Solarization alone or with organic amendments (biosolarization) is considered as an effective way of controlling soil-borne pathogens in Mediterranean climates, both for greenhouse crops ( González et al. , 1993 ; Guerrero et al. , 2010 , 2014 ), as well as in open field ( Tjamos & Paplomatas, 1988 ). Thermal increases in the soil during the disinfestation process have been shown to be particularly effective in controlling Verticillium , by drastically and sustainably reducing the inoculum ( Pullman et al. , 1981a ; Melero et al. , 1995 ; Pikerton et al. , 2000 ; Tamietti & Valentino, 2001 ; Goud et al. , 2004 ; Berbegal et al. , 2007a ; Yolageldi et al. , 2012 ), directly or by the effects of temperature on the OM biodecomposition or the antagonist micro­bio­ta ( Katan et al. , 1976 ; Tjamos & Paplomatas, 1988 ).

The critical threshold to eliminate 90% of V. dahliae microsclerotia inoculum on artificial media at constant temperatures of 38, 40, 42, 45°C required exposures times of 324, 275, 97 and 21 cumulative hours respectively ( Tamietti & Valentino, 2001 ). In the first year of our study, although exposure time (310 h) in the FSM+BR biosolarization treatment was slightly below the mentioned critical threshold (324 h) at 38°C, the treatment efficacy in inoculum inactivation could be related to the fact that V. dahliae microsclerotia on artificial media are more resistant to heat than those in natural solarized field soil ( Pullman et al. , 1981a ). According to Pullman et al. (1981a) , thermal inactiva­tion of V. dahliae microsclerotia required 14 days (336 h) at 37°C of constant temperature in moist soil and 28 days at 35-37°C of fluctuating temperature in solarized field soils in California. The thermal regime in the biosolarized soils of our first-year trial (2015-2016) exceeded that described as effective to inactivate V. dahliae and R. solani in solarized field soils in California. Although the number of cumulative hours above the thermal level that is considered to be lethal for the Verticillium inoculum was only reached at 15 cm in FSM and FSM+BB and at 30 cm in FSM+BB in the first cycle (2015-2016) of our experiment, Pikerton et al. (2000) indicated that the sublethal action of temperatures lower than the threshold considered to be lethal, or the fumigant effects of the amendments, could explain the drastic soil inoculum reduction after biosolarization in year 2015.

In the second crop cycle (2016-17), conversely, biosolarization treatments showed a much lower number of cumulative hours and were below the refe­rence threshold values for an effective control of V. dahliae microsclerotia. Temperatures exceeding 38°C were not achieved in the non- disinfestated control in either of the two years of biosolarization (2015 and 2016).

The effectiveness of biosolarization treatments in which thermal inactivation thresholds were not reached (FSM+BR in 2015-16; FSM, FSM+BB, FSM+BR in 2016-17), could be explained by the synergistic effect of other factors such as the increase and retention of volatile toxic compounds in the amended tarped soil, the anaerobic soil conditions and the increase in biological activity ( Gamliel, 2000 ; Stapleton, 2000 ; Blok et al. , 2000 ) and the effect of sub-lethal temperatures ( Davis & Sorensen, 1986; Tjamos & Paplomatas, 1988 ).

Previous works have shown that both Verticillium and Rhizoctonia differ in their sensitivity to heat. Inoculum population density of R. solani declined faster than V. dahliae in solarized soils ( Pullman et al. , 1981b ). In controlled laboratory conditions, Rhizoctonia was the first fungus to lose viability at constant temperatures above 39°C. At 42°C of constant temperature, the exposure time required to kill mycelia in agar medium was 33 h for R. solani and 44 h for V. dahliae , although R. solani showed a shorter recovery time than V. dahliae for restarting mycelial growth after exposure at high temperatures of 37-50°C.

Biosolarization treatments significantly reduced soil inoculum density of V. dahliae in soil and inocu­lum infectivity of both fungal pathogens compared to non-disinfestated soil in both years and presented significantly lower percentages of affected plants at the end of the crop in the case of V. dahliae . On the contrary, final crop percentage of R. solani infected plants in biosolarization treatments were not significantly different from the non-disinfestated control in both years, except the FSM+BR biosolarization treatment in the first year. The Rhizoctonia incidence in 2016-2017 (Table 5) did not appear to be directly related to the survival of the inoculum buried in the soil prior to disinfestation (Table 4); the reduction in the inoculum's survival with respect to the non-disinfestated control varied between 55.5% and 33.3% at the depth of 15 cm, and between 77.7% and 44.4% at 30 cm. Despite the reduction of soil inoculum infectivity of R. solani and V. dahliae after biosolarization, as estimated by the infectivity bioassays (Table 4), final crop incidence by R. solani was considerably higher than incidence by V. dahliae (Table 5). A possible explanation of the high final incidence of R. solani in the crop could be its greater saprophytic ability ( Bonanomi et al. , 2007 ), which would favour its growth with the use of easily assimilable nutrients generated after amendment de­com­position in the biosolarized soil treatments. The estimated inoculum density in the non-disinfestated soil for the two years of the present study (1.2 to 0.13 microsclerotia g ^-1 of soil) was low in comparison with the level found in artichoke crops in Levante, Spain (5 to 9 microsclerotia g ^-1 of soil) by Berbegal et al. (2007b) ; the incidence of Verticillium wilt (35.5% in 2015-2016 and 34.7% in 2016-2017) was also lower than in the trials carried out in Levante (almost 91%). However, the reduction in yield in the control with respect to the mean of biosolarization treatments was similar (30.1% in the Levante trial; 20.0% in 2015-2016 and 32.5% in 2016-2017 in our trials).

Although plants infection by Rhizoctonia was lo­­wer in the second season than in the first one (with the exception of treatment FSM + BR), the crop marke­table yield was lower in the second season than in the first one in all the treatments, with a more pronounced decrease in the non-disinfestated control than in the biosolarization treatments. This fact could be explai­ned by soil fatigue caused by repeated monoculture as described in other pathosystems ( Zydlik & Pacholak, 2008 ; Guerrero et al. , 2014 ). All biosolarization treatments significantly improved the marketable yield by 22-29% to 38-59% compared to the non-disinfestated control in the 2015-2016 and 2016-2017 crop cycles, respectively. The reduction of marketable yield in the second crop cycle compared to the first crop cycle was 18% for the average of biosolariza­tion treatments and 31% for the non-disinfestated control treatment, although the Vertici­­llium incidence was si­mi­lar in both years (Table 5) and that for Rhizoctonia did not increase in the second year. We consider such differences to be related to the effect of soil fatigue due to reiterated monocropping, as occurs in other intensive crops (Katán, 2005; Martínez et al. , 2011 ; Guerrero et al. , 2014 ).

In one solarization trial using crop cauliflower resi­dues and these with the addition of metam sodium, Berbegal et al. (2007a) obtained similar yields per plant when comparing cauliflower residues, so­la­rization alone and solarization combined with cauli­flower residues. In contrast, crop disease incidence by Verticillium was 80% in the cauliflower residues treatment, whilst in the treatments of solarization alone or solarization combined with the cauliflower residues, disease incidences were 30% and 38%, respectively.

In addition to the effect of biosolarization on the pathogens it would also be necessary to assess the effect that the organic matter and its decomposition at high temperatures has on the soil's physical and chemical characteristics. Our trials have shown that biosolarization is an effective mechanism, continued over time, to control artichoke soil-borne pathogens. In southeastern Spain, a zone where vegetables like artichoke are cultivated intensively, especially in repeated monocultures, biosolarization with different organic amendments can be recommended as an effective management strategy for the control of soil-borne fungal diseases and the improvement of soil fertility and crop yield.