When you hear about rainy or wet weather affecting lettuce fields, from where your lettuce is being grown, pay attention to the defect “Rib Discoloration.”  This defect is caused by an infection, the lettuce mosaic virus. The pathogen infects the ribs of lettuce coming from the soil it grows in.  Lettuce exposed to heat stress while growing have been known to exhibit symptoms of this defect as well.

From an inspector’s standpoint, do not overlook this defect.  At a quick glance you may think it is bruising.  But the midribs will not be damaged, broken or crushed in any way.  The discoloration will be visible, easily seen, affecting the interior of the midrib as opposed to the surface.  And trust me, this will quickly spread to decay.

In initial stages, rib discoloration appears as a creamy-yellow or light brown area on the inner surface of the midrib or lateral ribs of the leaf. At this stage the discolored area is usually less than one inch in length and about a quarter inch wide. The affected tissues are firm and not sunken, pitted or slimy. Frequently at this stage a slight discoloration is found on one or a few of the outer head leaves but may also occur on the inner head leaves. It is usually found on the area of greatest curvature of the leaves but may also occur closer to the butt of the head or farther out on the leaf on the lateral ribs or large veins.

In more advanced stages rib discoloration becomes reddish-green or blackish-brown in color. At this stage the discolored area may range from one to three inches in length and up to one-half inch in width. It is clearly visible on both surfaces of the leaf, and the affected tissues are sunken and occasionally cracked. Decay often follows advanced stages of rib discoloration.

Score as a defect, as damage when the aggregate area of Rib Discoloration exceeds 1 inch in length.   If a head had more than two leaves so affected, the appearance of the head would be damaged and scored as a defect, regardless of the length of the defect.