!,~^La AGRICULTURAL RESEARCH AND -EUCATION CENTER:
S-1 BELLE GLADE, FLORIDA
MIMEO REPORT BG71-10 APRIL 1971
VIRUS DISEASES OF LEAFY VEGETABLES
T. A. ZITTER1-
The principal virus diseases affecting leafy vegetables in the Everglades
are lettuce mosaic virus (LMV) and bidens mottle virus (BiMV). Lettuce and endive
varieties are susceptible.to both of.-these diseases. Other viruses occurring in
the state which have been reported to infect lettuce include ;cucumber mosaic virus
(CMV), sowthistle yellow vein virus (SYVY), and turnip mosaic virus (TuMV). How-
ever, at present the latter do..not appear to play any significant role in crop
loss. Work on the identification and control of virus diseases affecting leafy
vegetables is being done in cooperation with Dr. D. E. Purcifull of the Plant Virus
Laboratory, Gainesville. The purpose of this report is to summarize available in-
formation concerning' the two main virus diseases, and to account for losses experi-
enced this season.
Brief History, Host Range, and Characteristics For LMV and BiMV
Lettuce Mosaic Virus: Lettuce mosaic virus was first recognized and des-
cribed as a disease of lettuce in Florida in 1921. Two years later, the virus was
discovered to be seedborne in many varieties of lettuce. Today LMV is recognized
as a major disease of lettuce wherever it is grown. The virus is transmissible
to a fairly wide range of species belonging to 13 plant families. Lactuca (lettuce)
and Cichorium (endive) are the only two genera of commercial importance. The virus
is transmitted by aphids in a stylet-borne manner, meaning that an aphid can acquire
virus from an infected plant and transmit it to a healthy one in less than a
minute. Ten species of aphids are known to transmit the virus experimentally, but
the green peach aphid, 1Myzus persicae (Sulzer), appears to be the most important
vector. Insecticides do not prevent virus spread by alatae (winged) aphids.
Numerous studies have been made to establish the source of inoculum for this
virus. Infected seeds are the most important source of primary inoculum. The
significance of weed hosts maintaining the virus throughout the year is still not
clearly known. Because seed is the most important source of LMV, a clarification
of the terminology used in labeling seed is important. Regular seed lots can be
expected to carry between 1 to 4 percent or more infected seed in most ordinary
lettuce stocks. Mosaic tested or "indexed" seed (MT), is guaranteed to have less
than 1/10 of 1 percent of the seeds infected based on a minimum 10,000 seedling
count. Mosaic-free seed (MTO) does not contain infected seeds based on a 30,000
seedling count. "Mosaic tested" seed was originally used in California to reduce
the amount of mosaic, but even this was found o thavy to1 a lerance,
especially when aphid populations are high. G e alinas Valley now use
"mosaic-free" seed as much as possible.
AUG 31 1971
1/ Assistant Professor (Assistant Plant Pat .qoolt),Uvyyyefs|lorkidfl Plorida,
Institute of Food and Agricultural Scienc ^rirni url PzR .J and Education
Center, Belle Glade, Florida.
Bidens Mottle Virus: Bidens mottle virus (BiMV) was a previously undescribed
virus disease in Florida until its discovery near Gainesville in 1966. It is now
recognized to be widely distributed throughout the state. The two weed hosts
from which the virus was first recovered are Bidens pilosa L. (beggarticks or
spanishneedles) and Lepidium virginicum L. (Virginia pepperweed). The virus is
known to infect plants in 5 families, but with additional work more hosts are
likely to be found. The virus is transmitted by several species of aphids in a
Classification of Lettuce and Endive Varieties
It is estimated that there are 200 or more varieties of lettuce (Lactuca
sativa L.), which can be classified into five groups. The four main types of
lettuce grown in this area are summarized below with representative varieties:
1) Crisphead or "iceberg" type e.g., Minetto, Great Lakes.
2) Looseleaf or bunching type e.g., Grand Rapids, Slobolt, Black Seed
3) Butterhead type e.g., Bibb, Boston (White and Dark Green).
4) Cos or Romaine type e.g., Valmaine.
Varieties of all four types are known to be susceptible to LMV.
Endive (Cichorium endivia L.) is the other group of leafy vegetables which is
known to be susceptible to LMV. Two types of endive are distinguished on the basis
of leaf appearance. Common endive (e.g., Green Curled Ruffec) has narrowed, curled,
and finely divided leaves. The escarole type (e.g., Florida Deep Heart and Full
Heart NR 65) is the broad-leaved type of endive.
Endive is often referred to as "chicory", however as correctly used, chicory
refers to Cichorium intybds L., of which Witloof (Dutch for "whiteleaf") is the
most common variety. Chicory is also susceptible to LMV.
Host range studies were used locally to isolate and identify the virus present
in leafy material and in suspected weed hosts. The bulk of the samples were
checked in Gainesville by electron microscopy, host range, and by serology.
Seedborne LMV was detected by a dry seed assay method successfully used in
California. This consisted of grinding approximately 500 seeds by weight in a micro
mill for 10 minutes. The scarified seeds were then placed in a mortar and ground
additionally in the presence of a 4 ml buffer solution. The resulting slurry was
rubbed on leaves of Chenopodium quinoa Willd. (lambsquarters), which had previously
been given at 24 hr. dark treatment.
Results and Discussion
Unseasonably mild temperatures and limited rainfall during the fall and winter
growing season accounted for unprecedented aphid populations. Aphids were
particularly abundant during December and January, and accounted for the widespread
infection of leafy vegetables. Strong winds during this period caused additional
spread of winged aphids.
Virus recovery and identification: Accurate identification is an important
criterion in establishing the etiology of virus diseases affecting leafy vege-
tables. The symptoms produced by these viruses are so similar that they cannot be
distinguished by visual observation. The symptoms vary considerably and include
vein clearing, yellow mottling, veinal necrosis and bronzing, and pronounced
serration of the leaf margins. The plants are usually stunted if infected at an
early age. Symptoms will also vary depending on the varietal type, the age of the
plant at time of infection, and soil fertility.
Lettuce mosaic virus infection was general among most of the leaf-types grown
in the area; however, not all of the types were checked for virus. In Table 1
are shown some of the plant materials which were indexed for virus, and the parti-
cular virus recovered.
Table 1. Recovery of viruses from material sampled from the Belle Glade area.
No. of Samples No. of Samples with:
Virus Source Indexed LMV BiMV
Crisphead varieties 2 1 1
Boston 2 1 1
Leaf lettuce 1 1 -
Valmaine 6 6
Escarole 11 2 9
Endive 2 -- 2
Bidens pilosa L. 2 -- 2
Lepidium virginicum L. 4 -- 4
The recovery of LMV from Lactuca and Cichorium varieties was expected, since
all have been reported to be susceptible. Of particular interest was the isola-
tion of BiMV from lettuce types as well as from escarole and endive. Although
preliminary and based on limited samples, escarole appears to be particularly
susceptible to BiMV.
Growers for some time have been concerned with the poor growth and color
and the yellowing of lower leaves in escarole, usually observed during the spring
months. Several attempts were made to recover virus from plants showing this con-
dition; however, results were negative. Plants which showed obvious mosaic
symptoms, either with or without yellowing of the lower leaves had virus present
(BiMV). Thus results at this stage are inconclusive, and additional work is needed
to clarify this problem.
The recovery of BiMV from Lepidium and Bidens confirms that these two weeds
act as a major source of inoculum for this area.
Recovery of LMV from seed: Since previous workers have reported that seed
was the major source of LUV, studies were made to determine, 1) how readily LMV
can be recovered from seed, and 2) which seeds) could have been a major factor in
supplying inoculum. The dry seed assay method as used in California has been shown
to be as effective in detecting virus as the usual method of growing out the seed-
lings for visual observation.
The results of this study (Table 2) indicated that LMV is readily recovered
from seed, especially from those which have no prescribed tolerances. The tech-
nique is an all or none proposition and does not tell the actual percent of seed
with mosaic. By far the most common source of LMV appeared in the looseleaf
lettuce types where 4 of the 5 samples checked were positive. Slobolt variety was
also checked by the conventional seedling method and gave positive results. The
fact that looseleaf varieties were heavily infected was also confirmed by grower
observations. All of the seeds tested were used during the current season except
for Iinetto (No. 22), which was held over from the 69-70 season. Virus could still
be recovered from this sample.
Table 2. Recovery of LMV from seeds using the dry seed assay method.
Seeds tested Code No. No. of tests Seed Source Results
Abbott & Cobb Positive
,Minetto 22 1 (1969 seed)
Black Seeded Simpson 12 1 Ferry Horse Negative
Slobolt 5 2 Harris Positive
Grand Rapids 20 1 Abbott 5 Cobb Positive
Grand Rapids 6 1 Ferry Morse Positive
Grand Rapids 13 1 Harris Positive
Bibb 7 1 Niagara Negative
Dark Green Boston 8 1 Ferry Morse Negative
White Boston 14 2 Asgrow Positive
Summer Bibb 21 1 Harris Negative
Valmaine 15 2 Ferry Morse Positive
Full Heart NR65
Control measures: Since both LMV and BiV are aphid-transmitted, stylet-
borne viruses, their spread cannot be controlled by insecticides. Thus the most
effective control measure is the reduction of available inoculum. In the case of
LMV, this means the use of "mosaic-free" or, if not available, "mosaic-tested"
seed. This will greatly reduce the amount of initial inoculum and thus the amount
of secondary spread. However, interplanting of non-indexed seed, such as loose-
leaf or any other type, places any "mosaic-free" seed in jeopardy. Cultural
management plays a big role in reducing the amount of virus spread. Abandoned
lettuce fields should be destroyed as soon as possible to eliminate them as a
reservoir for both aphids and virus. Elimination of Lepidium virginicum L. and
Bidens pilosa L. from ditchbanks surrounding lettuce fields should be done to
reduce infection of leafy vegetables with bidens mottle virus.