Orobanche spp. (orobanches)

The orobanch (broomrapes) are parasitic plants, devoid of roots and chlorophyll, whose nutrition is totally dependent on their hosts. It has been listed more than 150 species. They attack many plants, whether they are cultivated or not, especially Solanaceae, Fabaceae and Cucurbitaceae, on which they can cause significant damage. The most harmful species are mainly located in the Mediterranean basin, south-west Asia and some other regions whose climate is close to the Mediterranean type (western Australia, California ??).

In France, the orobanche rower (Orobanche ramosa L. Phelipaea ramosa C.A. Meyer), a species with the widest spectrum of hosts, is in recrudescence, especially on hemp, rapeseed and on another olanacea, tobacco. To our knowledge, it does not currently seem to be a big problem for field tomato crops. Note that several other species of Orobanche have been described on tomato; O. aegyptiaca Pers., O. cernua Loefl ??

The appearance of tufts of white, yellow or purple at the foot of the plants characterizes the parasitism of the orobanches (Figures 1 and 2). These tufts correspond to the flower stalks which can have a height of one to several tens of centimeters. These stems bear pale blue to purplish blue flowers in O's case. ramosa, which will produce capsules containing 300 to 500 seeds each (seeds measuring 0.2 to 0.4 mm long).

In fact, the parasitism of orobanches concerns mainly the roots of tomatoes on which they set up suckers, then tentacle anchoring roots, which allow them to absorb the mineral elements essential for their growth. The plants are weakened and are therefore smaller and stunted. In the long term, fruit yields are reduced. In addition, these plants are unable to regulate their stomatal transpiration during the dry period, so they maintain their host in chronic water deficit.

Orobanchae are preserved in the rather superficial layer of the soil by means of their tiny seeds, which perennialize for several years; 10 and more. They can also be maintained and multiply in the soil by means of numerous cultivated hosts (example of vegetables sensitive to O. ramosa: aubergine, lettuce, pepper, Cucurbitaceae, bean, peas, carrot, celery, fennel) and certain weeds (Capsella bursa-pasteuris, Solanum spp., Amaranthus spp.). In the presence of sufficiently moist soil and stimulated by root exudates, the seeds germinate and attach to the roots of their host. Subsequently, the orobanche enter the latter and set up a sucker in connection with the xylem of its host, allowing them to ensure their carbon, mineral and water to the detriment of plants. It is at this moment that the first underground growth of the stem begins. This is not slow to emerge and to form the flower stalk, which blooms thereafter. In the long term, several tens of thousands of seeds will be produced per Orobanche plant. These can be scattered by irrigation water or as a result of floods, wind, animals, agricultural implements, polluted seed lots ??

The fight against these highly polyphagous parasitic plants is often very delicate because of their enormous potential for conservation and dissemination. We advise you to implement the following measures:

to pluck or destroy by all means parasitic plants before their flowering;

- in the presence of an early outbreak, treat orobanches and their hosts with a herbicide such as glyphosate to prevent plants from producing seeds that will return to the soil;

-optimize the fight against certain weeds (brassicaceae ??) that can replace the tomato and perpetuate these parasitic plants in the absence of sensitive crops;

-in some countries soil disinfection with a fumigant is envisaged (methyl bromide). Solarization would destroy part of the seed stock in the first few centimeters of soil;

-The deep burrowing of the seeds is sometimes advisable because it removes them from future tomato roots which will be parasitized only belatedly. This practice is controversial, however, as it contributes to their conservation;

the increase in nitrogen fertilization would reduce infestations by reducing their germination and fixation capacity on their hosts. Ammoniums are more inhibitory than nitrates;

- avoid contaminating new plots with soiled agricultural equipment. For this, we suggest that you always work the infested plots last and wash the material well afterwards.

As an example, let us point out that a resistance to O. aegyptiaca, monogenic, dominant and controlled by the "Ora" gene, was highlighted and worked on tomato in Egypt.

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• Source :
 
• D  Blancard (INRA  France)