First report of a root and crown disease of the invasive weed Lepidium draba caused by Phoma macrostoma

Authors: Caesar, Anthony; Lartey, Robert; Caesar, Thecan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2011
Publication Date: 1/1/2012
Citation: Caesar, A.J., Lartey, R.T., Caesar, T. 2012. First report of a root and crown disease of the invasive weed Lepidium draba caused by Phoma macrostoma. Plant Disease. 96(1):145.

Interpretive Summary: In surveys to detect potential biocontrol agents of the deep-rooted and aggressively expansive invasive weed white top or hoary cress, the fungus Phoma macrostoma was among the root pathogens cultured from roots of noticeably chlorotic plants. The fungus was isolated along with other soilborne pathogens from roots with lesions that appeared as fissures. The pear-shaped fruiting bodies of the fungus, from which masses of the tiny oval-shaped spores are produced are black and have large openings in their necks. Inoculating roots, planting them and allowing shoots to emerge, reproduced the chlorotic or yellowing symptoms of the plant top-growth. This fungus species is being tested for use as a biological control agent of weeds on lawns and some agricultural weeds such as wild mustard. It has been considered a weak or wound-associated pathogen, and in our studies showed the ability to cause disease on its own when roots were wounded. We consider it’s presence to be a further sign that a variety of pathogens against a perennial have accumulated over time and such sites would be good locations for future early release sites of insect natural enemies against white top.

Technical Abstract: The exotic rangeland perennial Lepidium draba occurs as a noxious weed in 22 states, mostly in the western U. S. Because chemical control measures against this invasive perennial have not achieved satisfactory results, biological control is being pursued. While inventories of arthropods that feed on L. draba have been established, little is known of soilborne pathogens of L. draba for possible use as biological control agents. To address this deficiency we have surveyed for diseases of L. draba in both the U. S. and Eurasia to identify and test potential biocontrol agents. In intensive surveys for soilborne diseases in a single infestation that is more than 20 years old in a cattle pasture in south central Montana, several chlorotic, stunted plants were noted. Roots of such chlorotic plants which exhibited elongated fissures from which other soilborne fungi were isolated also had numerous prominent pycnidia embedded in the crown tissue above such lesions. Examination with a dissecting scope revealed large ostioles, made evident by the wide concave inversions in the short necks of the pycnidia. Culture of root tissue on PDA resulted in whitish, becoming pale grey colonies, with a dull peach to reddish tinge at the margins, with abundant single pycnidia. Conidia in vitro were mainly unicellular, of variable shape, subglobose to ellipsoidal with several guttules with an average size of 6 x 2.5 2m. These morphological traits are characteristic of Phoma macrostoma, which is regarded as a weak or wound pathogen, but which has been investigated extensively for use as a biological control agent of various agricultural weeds (1) including wild mustard. The internal transcribed spacer region of rDNA was amplified using primers ITS1 and ITS4 and sequenced. BLAST analysis of the 575-bp fragment showed a 100% homology with a sequence of P. macrostoma (GenBank Accession No. DQ474091). The nucleotide sequence has been assigned GenBank Accession No. HM755951. Pathogenicity tests consisted of making 4 1.4 mm-diameter holes in 5 NaOH (0.1%)-sterilized root sections of L. draba and pipetting ca 50-100 2l of a 106 cfu/ml conidial suspension into the incisions, incubating the inoculated roots at lab temperature (20-25º C) overnight and planting the root sections, one per pot, in an artificial greenhouse potting mix and placing the pots in the greenhouse at 20-25º C. Controls were 5 root sections which were treated similarly except that sterile water was injected. The experiment was repeated. After 10 days, shoots that grew from inoculated roots were chlorotic and were shorter than those produced from control roots. P. macrostoma was isolated from tissue of inoculated roots which became blackened distal to the inoculation points. To examine the host range of P. macrostoma on other brassicae species, crowns of 2-week old seedlings of radish, broccoli, cauliflower, broccoli raab, turnip, kohlrabi, cabbage, Chinese cabbage, mustard greens and canola were injected with 0.5 ml a 106 cfu/ml conidial suspension. Plants were grown in the greenhouse at 20-25º C for 4 weeks after inoculation and examined for symptoms. The experiment was repeated twice. Blackened root tissue, with slight chlorosis occurred only on roots of radish and crowns of broccoli, from which P. macrostoma was reisolated. This the first report of a disease of L. draba caused by P. macrostoma. Reference: (1) Bailey, K.L. et al. 2001. US Patent Application Serial No. 60/294,475, Filed May 20, 2001.