Symptoms and Signs
Early symptoms of Phoma basal rot on romaine lettuce consist of yellowing and wilting of lower leaves. Affected plants may later show an uneven appearance because one side of the foliage grew normally while the diseased side of the plant remained stunted and short. As plants mature, the entire plant becomes stunted and eventually wilts and collapses. The wilting and collapsing symptoms are somewhat similar to lettuce drop and Botrytis crown rot.
Examination of affected plants reveals distinct, black cavities (sunken areas) on the crown and upper taproot. The cavities extend far into the crowns and roots, resulting in extensive weakening of the plant; these plants could be broken off easily at ground level. The black cavities are characteristically firm and lack any sign of fungal growth, sclerotia, or spores. This disease is primarily found on romaine; other lettuce types are susceptible but typically are infected at low incidences.
Comments on the Disease
Phoma exigua is a soilborne fungus. Phoma basal rot was first documented in California in 2000 and caused significant losses in romaine plantings. The disease has been found in coastal counties, but has not yet been confirmed in San Joaquin Valley or desert regions. Little is known about the epidemiology of this disease.
Avoid planting romaine cultivars in fields having histories of this problem.
- Apply protectant fungicides after thinning the crop to reduce Phoma basal rot problems.
- Apply fungicide before plants become too large and direct applications to the base of the young plants.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Quadris)||12.3–15.4 fl oz||4||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)|
|COMMENTS: Alternate with a fungicide, such as sulfur, that has a different mode-of-action group number. See label for special phytotoxicity precautions.|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxamide (7)|
|COMMENTS: Do not make more than two applications per season.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number.|