INFECTION OF TOMATO STEMS BY BOTRYTIS CINEREA PERS. EX. FR.
Experimental work has shown that the stems of young tomato plants are highly resistant to attack by B. cinerea but become susceptible with increasing physiological age. Resistance to surface infection of wounds involves restriction of mycelial growth rather than inhibition of the spore germination and penetration phases of parasitism.but in the case of vascular attack, as is indicated later, the germination of spores may be inhibited. The resistance of physiologically young tissue is reflected in the in vitro growth of the pathogen in tissue extracts and excised stem internodes. In the former case addition of 1% glucose to the extract of resistant tissue raises the level of growth of the fungus to near that obtained in the extract of susceptible tissue. Resistance in the growing plant is readily broken down where there is penetration by a saprophytically based mycelium. Several factors, such as high soil moisture and restriction of root growth, have been shown to accelerate transition from the resistant to the susceptible phase.
Infection at deleafing wounds occurs either by hyphal penetration of the surface or from spores drawn in to the cut ends of the xylem vessels. It has been shown experimentally that dry spores applied to a wound surface are not drawn into the vessels. If, however, vascular pressure is raised above that of the atmosphere by stopping transpiration, the spores on the wound surface become suspended in exudate and are drawn into the vessels when transpiration is resumed and the vascular pressure again becomes sub-atmospheric. It is suggested that this may be the mechanism involved under the conditions prevailing in commercial glasshouses.
Although spores can pass for distances of several centimeters along vessels in petiole stubs, they rarely enter to depths of more than a few millimeters where the petioles have been cut or broken off close to the stem. Although the spores in the vessels of resistant plants may germinate to give a vesicle or a very sparse mycelium, they often do not do so. In the case of susceptible plants a high proportion of such spores germinate and the break out of hyphae from the vessels into the surrounding tissues has been demonstrated.
When infection of a young resistant plant occurs, it has been shown that the pathogen may remain latent for a considerable period - time lapses of up to 11 weeks between inoculation and the appearance of lesions have been recorded. The form of the latent phase, whether germinated or ungerminated spores in the vessels, or a sparse mycelium below the wound surface, is not yet known with certainty although available evidence points to the vascular infection.
Application of thiram and captan dusts immediately following deleafing has not provided effective control of infection. Good results have been obtained with thin pastes of Bayer 5114, thiram and captan applied by brush to the deleafing scars at this time but this cannot be regarded as a practicable method of control under commercial conditions. Modifications in crop management based on the increasing knowledge of the biology of the pathogen would appear to be a more promising line of investigation in attempts to combat this form of Botrytis attack.