Bean common mosaic is a viral disease caused by several different virus strains. Close cousin to bean yellow mosaic and clover yellow vein virus, bean plants can be unfortunate enough to be infected with all three at the same time.
What’s really fascinating about this disease is that there are two different sets of symptoms that may occur.
Symptoms of bean common mosaic
Bean plants infected with these viruses may come down with bean common mosaic or bean common mosaic necrosis.
Bean common mosaic is very common in California. It appears as mosaic patterns of light and dark green on the leaves. Puckering, blistering, rolling, and downward cupping are also common symptoms. Plants infected while young will also be stunted.
Bean common mosaic necrosis has not been seen in California since its first sighting in 1996, but you’ll want to be on the lookout, just in case. Symptoms include small, reddish-brown spots on the leaves. Nearby leaf veins become brown or black and this necrosis (death) then spreads to the phloem and throughout the plant, ultimately killing it. It you take a cross-section of an infected stem or pod, you will see reddish streaking in vascular tissue. These symptoms look very similar to black root rot and Fusarium wilt, but neither of these conditions cause streaking in the pods.
In both cases, leaves may also be smaller than normal, and blossoms and pods may be deformed. Which set of symptoms your plants will exhibit depends on which virus is involved, and whether or not your plants have dominant or recessive genes, or if one gene is present at all. Plants with the dominant gene are resistant to common mosaic, but hypersensitive to common mosaic necrosis. Some bean varieties, which lack the gene altogether, have symptoms which could fall under either category. Symptoms of bean common mosaic are most likely to appear when temperatures are between 68 and 77°F.
Bean common mosaic transmission
The bean common mosaic virus overwinters in infected seeds and weeds. The virus is most often transmitted on pollen and by aphids. It can also move from plant to plant on clothing, tools, and plant debris.
The best way to avoid introducing this virus into your garden is to:
Resistant bean varieties
According to UCANR, these varieties are resistant to one or more strains of this virus:
Once these viruses are in your soil, they are difficult to get rid of - it is far better to start with clean seeds and do what you can about those pesky aphids.
You can grow a surprising amount of food in your own yard. Ask me how!