Suppression of Botrytis Blight of Begonia by Trichoderma hamatum 382 in Peat and Compost-Amended Potting Mixes L. E. Horst, J. Locke, and C. R. Krause, U.S. Department of Agriculture, Agricultural Research Service, Applica- tion Technology Research Unit, Wooster, OH 44691; R. W. McMahon, Ohio State University, Agricultural Techni- cal Institute, Wooster 44691; and L. V. Madden and H. A. J. Hoitink, Ohio State University, Department of Plant Pathology, Wooster 44691 zianum T39 (14), T. harzianum T22 ABSTRACT (30,44), T. harzianum T-203 (46,56), and Horst, L. E., Locke, J., Krause, C. R., McMahon, R. W., Madden, L. V., and Hoitink, H. A. J. T. virens G6 (7,24). To our knowledge, 2005. Suppression of Botrytis blight of begonia by Trichoderma hamatum 382 in peat and com- none of these isolates have been tested for post-amended potting mixes. Plant Dis. 89:1195-1200. activity on begonia against Botrytis blight. Isolates of the Trichoderma spp. listed Inoculation of an industry standard light sphagnum peat potting mix with Trichoderma hamatum above can be natural inhabitants of bark, 382 (T382) significantly (P = 0.05) reduced the severity of Botrytis blight, caused by Botrytis peat, or composts used widely as ingredi- cinerea, on begonia plants grown in a greenhouse. In data combined from three experiments, the ents in potting mixes in the ornamentals degree of control provided by T382 did not differ significantly (P = 0.05) from that provided by weekly topical sprays with chlorothalonil. In addition, T382 significantly (P = 0.05) increased industry (11,22,32,54). However, specific shoot dry weight and salability of flowering plants. Incorporation of composted cow manure biocontrol agents such as the Trichoderma (5%, vol/vol) into the light peat mix also significantly (P = 0.05) decreased blight severity while isolates listed above that can induce sys- shoot dry weight and salability were increased. Blight severity on plants in this compost mix did temic resistance in plants do not consis- not differ significantly (P = 0.05) from that on those in the light peat mix inoculated with T382. tently colonize potting mixes before plant- Finally, T382 and chlorothalonil did not significantly (P = 0.05) affect blight severity, shoot dry ing of potted crops (28,31). As a result, weight, or salability of plants grown in the compost mix. Spatial separation was maintained in most compost-amended potting mixes do begonias between the biocontrol agent T382 and the pathogen. It was concluded, therefore, that not naturally reduce the severity of foliar the decrease in disease severity provided by inoculation of the peat mix with T382 most likely diseases (28,31,38,57). For example, only was due to systemic resistance induced in begonia against Botrytis blight. The suppressive effect one of 79 different batches of natural com- of the compost mix against Botrytis blight was unusual because composts typically do not pro- posts tested induced ISR in radish against vide such effects unless inoculated with a biocontrol agent capable of inducing systemic resis- bacterial leaf spot caused by Xanthomonas tance in plants to disease. campestris pv. armoraciae (31). Thus, inoculation with an ISR-active biocontrol Additional keywords: Begonia hiemalis, biological control, ISR, marginal effects analysis agent is essential for consistent efficacy against foliar diseases of plants grown in potting mixes. Botrytis blight, caused by Botrytis cine- fungicides labeled for floricultural crops Several reports show that the microbial rea L., is an economically important dis- such as vinclozolin, benzimidazoles, dicar- carrying capacity of the organic fraction in ease of begonia and other greenhouse boxamides, and diethofencarb (6,16,21). potting mixes affects suppression of Py- crops, where it can cause lesions on flow- Recently, biocontrol agents were added to thium and Phytophthora root rots ers, leaves, and stems during the entire the list of commercial control strategies to (4,22,47,48). For example, slightly decom- cropping cycle (12). The inoculum of the improve control of this disease (18). posed, light fibrous (H2-3 on the Von Post pathogen is widely distributed in green- Bacterial and fungal biocontrol agents decomposition scale) sphagnum peat (40) houses (26). The disease is particularly have been proposed as topical sprays for supports the activity of biocontrol agents severe under low light and high humidity control of Botrytis blight on greenhouse and suppression of Pythium root rot (3,4). greenhouse conditions (21,25). The great- crops (6,15,27,29,34). Combinations of In contrast, highly decomposed, dark est losses occur during transit and storage biocontrol agents have been proposed as sphagnum peat (H4) harvested from deeper (25). Standard control procedures for well (17). The disadvantage of topical and older layers in peat bogs does not pro- greenhouse crops depend heavily on cul- treatments is that the introduced biocontrol vide these beneficial effects (3,4). Recent tural practices and humidity control, but agent most likely will come in direct con- reports indicate that ISR induced by also on timely applications of protective tact with fungicides applied to the foliage. rhizosphere microorganisms in plants also fungicides (18,21). Unfortunately, B. cine- This implies that it would have to be resis- depends on soil organic matter quality. For rea has developed resistance to several tant to such compounds. A solution to this example, Pharand et al. (38) reported that potential problem is the use of rhizosphere the degree of protection provided by Py- microorganisms that induce systemic resis- thium oligandrum in tomato against crown Corresponding author: Harry A. J. Hoitink tance (ISR) in plants and thus provide rot caused by Fusarium oxysporum f. sp. E-mail: [email protected] control of root as well as foliar diseases radicis-lycopersici was improved by (39,41). Fungal as well as bacterial amendment of a sphagnum peat mix with Mention of a trademark, proprietary product, or rhizosphere microorganisms may induce composted pulp and paper mill residues. vendor does not constitute a guarantee or warranty of the product by the USDA, and does not imply its this systemic effect in plants (14,20,55). Furthermore, suppression of Phytophthora approval to the exclusion of other products or Isolates of several Trichoderma spp. can leaf blight and stem dieback of cucumber vendors that also may be suitable. reduce the severity of foliar diseases of provided by T382 in a sphagnum peat mix plants when applied as seed or transplant was enhanced by amendment of the mix Accepted for publication 13 June 2005. treatments, presumably by inducing ISR in with composted dairy manure (28). The plants. They include Trichoderma asperel- severity of the Phytophthora-induced dis- DOI: 10.1094/PD-89-1195 lum T-203 (20,55), T. hamatum GT3-2 (9), ease observed in the latter work did not © 2005 The American Phytopathological Society T. hamatum T382 (T382) (19,31), T. har- differ between that provided by a drench Plant Disease / November 2005 1195 with the fungicide metalaxyl and that ob- ceived from Sylvan Bioproducts, Inc., were dislodged with a glass rod. The co- tained with the fortified compost-amended Cabot, PA) at a rate of 120 g m-3 mix to nidial concentration was determined by mix. Apart from biocontrol agents, the establish an initial population density of 2 using a hemacytometer, and the concentra- quality of organic matter in field soil also × 105 CFU T382 g-1 dry weight mix. Con- tion was adjusted to 2.5 × 105 ml-1 by add- seems to play a role in systemic induced trol transplant mixes were not inoculated ing distilled water as needed. All plants resistance (47,52). Thus, expression of with T382. All mixes, including the con- were then sprayed to runoff with this in- such activity in potting mixes may depend trols, were incubated at 25°C for 7 days oculum. During preliminary experiments, on specific microorganisms present in the before planting. The population of T382 in it was determined that in vitro growth on substrate as well as the chemistry of the the control and inoculated potting mixes PDA of the isolate of B. cinerea used in organic matter in the mix. was monitored on two samples per treat- this work was inhibited at a concentration The specific objectives of this work ment using a triplicate dilution series on a of 100 µg a.i. chlorothalonil per ml of PDA were to determine whether: (i) inoculation Trichoderma selective medium (10). All medium. The pathogen, therefore, was not of T382 into a light sphagnum peat or a treatments were sampled immediately after resistant to the fungicide used in this work compost-amended potting mix can reduce mix preparation, at planting, and at 30 and (56). Botrytis blight severity was deter- the severity of Botrytis blight of begonia; 56 days after planting. Colonies of T382 mined when symptoms developed at 4 to 7 (ii) amendment of the light sphagnum peat were counted after 10 days of incubation at days after inoculation and at 3-day inter- potting mix with compost affects efficacy 24°C. The identity of three presumed T382 vals thereafter until 56 days after planting of T382 in control of Botrytis blight; and colonies per potting mix sample was veri- on the basis of percentage of total plant (iii) the effect induced by T382 is systemic fied by light microscopy. Phialides pro- surface displaying symptoms, using a scale in nature. duced by T382 examined under a light of 0 to 100 in 10-unit increments where microscope served to verify the identity of 0% = symptomless, 10% = 10 percent of MATERIALS AND METHODS T. hamatum according to Bissett (2). Con- total plant displaying symptoms, and Preparation of potting mixes. A light firmation of three putative isolates of T382 100% = death of the plant.