NEWSLETTER ARTICLES

Vegetables and Plant Pathology

HEAD ROT IN BROCCOLI

Head rot or bead rot of broccoli, also called "pin rot," has become a major problem in many Santa Maria broccoli fields this winter. Yields have been severely reduced, and some fields have been abandoned due to the lack of blemish-free heads. The rot may start anywhere on the head. A common occurrence, however, is that the first beads affected are at the center of a developing floral branch. These are the most immature tissues in the head. One to several beads become greenish-black or watersoaked, then turn brown to black as the tissues deteriorate. The damaged tissues may remain confined to a few beads or the rot may spread down into the floral stems and across the beads to encompass a spot 1/4 to 1/2 inch in diameter. The rot can be found any time after head formation begins. Broccoli floral heads from 3/4 inch to harvest maturity may be damaged. If the rot is present early, the head usually becomes deformed because the damaged beads cease growing, and the surrounding beads grow into the gap.

Isolations made from damaged tissue show the presence of several organisms: Alternaria brassicae is the primary fungus that has been isolated from discolored spots that show a greenish-black fuzzy growth. This fungus is also responsible for causing Alternaria leafspot in broccoli, pak choi, Chinese cabbage, and cauliflower.

Another primary cause of head rot in broccoli is a complex of bacteria. When only bacteria are present, the initial infection appears as watersoaked beads, which appear darker in color than surrounding beads. Affected beads disintegrate to a mushy mass when handled and examined.

The primary pathogens in the bacterial head rot of broccoli are Pseudomonas fluorescens and Pseudomonas marginalis. Both bacteria produce a surfactant-like substance (viscosin) and pectolytic enzymes. The surfactant allows the bacteria to penetrate the waxy cuticle of the flower buds after which the pectolytic enzymes are able to attack and digest the broccoli floral tissues.

Broccoli head rot is most active during the late fall and winter months in the Santa Maria and Lompoc Valleys. The almost constant rainy, foggy, humid conditions with temperatures in the 40º to 60ºF degree range of October, November, December 1997, and now January of 1998, have apparently provided ideal conditions for head rot fungi and bacteria to operate. At present, I have no good suggestions for control. Plant health product tests, run during the fall of 1995 using several chlorothalonil and copper formulations, did not provide effective control of this problem. Plans are now underway to initiate another series of tests. Results of these trials will be published when available.

LETTUCE MOSAIC VIRUS

The following excellent review of lettuce mosaic disease was prepared by Steve Koike, Farm Advisor, Monterey County. The Arroyo Grande, Santa Maria and Lompoc Valleys each have a number of potential LMV reservoir sites. Surveys have determined that LMV is present in several native and ornamental plants in each valley. Even though LMV has not been a problem in recent years, small isolated outbreaks indicate that the virus is around, and the potential for major problems is only a careless, nonvigilant moment away. This article presents a good reminder that our guard against this disease should not be relaxed!

Historically, lettuce mosaic has been the most important virus disease of lettuce in California. Prior to the 1960s, the lettuce mosaic virus (LMV) caused widespread infection and crop loss in the Salinas Valley. Today, a well established, diligent, integrated control program maintains lettuce mosaic incidence at low levels. However, lettuce producers should continue to be aware of the potential for LMV outbreaks.

Young lettuce seedlings that are either diseased at emergence (from seedborne infections) or infected at an early developmental stage usually show significant yellowing (chlorosis), stunting, and deformity of leaves. Leaf texture may be leathery, and leaf margins can be excessively "toothy" or jagged. Actual mosaic symptoms, consisting of yellow and green mottling and uneven green coloring of the leaf, may not always be evident. Plants infected at this early stage will rarely develop heads (iceberg or butterhead cultivars) or large frames (leaf and Romaine types).

Chlorosis, stunting, and leaf deformity can be evident on plants infected at later stages, but such symptoms are usually less severe. On larger plants the leaf margins roll backwards away from the center axis of the plant. This is a characteristic feature that is particularly evident on iceberg cultivars. Leaf margins again are toothy or jagged, and developing heads can be deformed. If infected at later stages, the plants can form fairly normal heads and sizable frames; such plants, however, may be chlorotic or faded green. The mosaic symptom (yellow and green mottling) is often absent on infected mature plants. On young and old plants, brown necrotic (small areas of dead tissue) lesions or spots can develop. Formerly attributed to severe strains, researchers have found that necrotic spotting can occur with probably most or all LMV strains found in the coastal valleys and hence is not a unique characteristic of particular isolates.

Other viruses can cause similar symptoms in lettuce, and therefore identification of virus problems based on symptoms alone is not advisable. In general, turnip mosaic virus usually causes more of a spotting symptom in which small (less than 1/4 inch), irregular, yellow spots develop on lettuce leaves. If infected at a young stage, plants can be severely stunted. Beet western yellows usually does not result in significant plant stunting or head deformity, but does cause the outer leaves to become bright yellow, with only a few major plant veins in these leaves remaining green.

LMV is a member of the Potyvirus group of plant viruses. LMV is seedborne in lettuce, able to infect a number of cultivated and weedy plant species (see host list in this article), and is transmitted by aphids (in a noncirculative, non-persistent manner). Like most viruses, this pathogen does not survive in the soil once infected plant residue is buried or decomposed. However, intact infected plants in harvested fields can still act as reservoirs for the virus.

A productive partnership between University of California campus researchers, U.C. Cooperative Extension personnel, and lettuce industry growers resulted in a management program that successfully keeps lettuce mosaic at minimal levels. County ordinances help enforce the first four aspects of the integrated program outlined below.

(1) Seed assays for LMV. All seed to be planted in coastal counties is to be tested for seedborne-LMV. Researchers found that a zero in 30,000 seed infection threshold is the key for LMV control in the coastal valleys (if no infected seed are found in a 30,000 lettuce seed sample serologically tested by the enzyme-linked immunosorbent assay (ELISA), the seed lot is approved for planting). Infection thresholds differ according to the epidemiological aspects (how the disease develops) of any particular area. For example, in parts of Europe the testing threshold is zero in 2,000 seed. The nature of the lettuce acreage and system in Europe allows the industry there to tolerate much more LMV-infected seed than the growers in the coastal valleys.

(2) Weed control. Because weeds can be a significant reservoir of the virus and the source from which aphids obtain the virus, weeds must be regularly controlled and removed in the lettuce production areas.

(3) Plow-down of old lettuce plantings. Old, infected lettuce plants, like weeds, are a source of virus. Aphids can pick up the virus from these old plants and readily transport it to younger, nearby lettuce plantings. It is important to plow down, in a timely manner, old plantings once they are harvested.

(4) Lettuce host-free period. To help prevent the continuous, year-to-year buildup of LMV, a host-free period is enforced for two weeks in December. This step is effective because LMV is an obligate pathogen and cannot survive in nature without living plant hosts. The ban on lettuce production in this winter period helps reduce the amount of virus that would "bridge" over from one season to the next.

(5) Resistant lettuce cultivars. While not used extensively in California, cultivars resistant to LMV are available.

(6) Aphid control. Clearly, aphid control is helpful in slowing LMV spread. However, spraying for aphids does not prevent the transmission of LMV because aphids can transfer the virus to plants before the insecticides act to kill the insects.

Outbreaks of lettuce mosaic occurred in the Salinas Valley and other coastal areas in the late 1980s and early 1990s, prompting concern about a possible breakdown in the control program due to the development of new strains of LMV. However, researchers found that LMV strains collected during this time period were not significantly different than "type" strains of LMV (type strains are considered the typical, generic form of the virus pathogen). When the collected strains (including so-called severe strains) and type strains were inoculated onto a series of lettuce cultivars, all plants exhibited identical symptoms. When collected strains (including severe strains) and type strains of amino acid sequences found in the proteins that make up the coat of the virus), all strains were closely related and no evidence for new strains was found. Those disease outbreaks now appear to be related, in part, to perennial plantings of infected ornamental plants (such as Gazania species) and aphid movement.

In conclusion, lettuce mosaic continues to be well managed in the coastal valleys. The multi-faceted control program is an excellent model for how plant diseases can be managed by basing control strategies on the biology of the pathogen, integrating those strategies into industry practices, and implementing those strategies on an industry-wide basis.

No common names

	Carduus broteroi
	Chenopodium amaranticolor
	Chenopodium quinoa
	Cicer yamashitae
	Lactuca virosa
	Lactuca livida
	Nicotiana benthamiana
	Nicotiana clevelandii
	Rumex britannica (=R. orbiculatus)
	Urospermum picroides

Agronomic plants

Safflower	Carthamus tinctorius
Chickpea, garbanzo bean	Cicer arietinum
Escarole	Cichorium endivia
Endive	Cichorium endivia
Witloof chicory	Cichorium intybus
Lettuce	Lactuca sativa
Pea	Pisum sativum
Spinach	Spinacia oleracea
New Zealand spinach	Tetragonia expansa

Ornamental plants

Love-lies-bleeding, tassel flower	Amaranthus caudatus
Aster	Aster species
China aster	Callistephus chinensis
Shasta daisy	Chrysanthemum maximum
Lisianthus	Eustoma grandiflorum
Gazania	Gazania spp.
Globe amaranth	Gomphrena globosa
Sweet pea	Lathyrusodoratus
Trailing african daisy	Osteospermum fructicosum
Cineraria	Senecio cruentus
African marigold	Tagetes erecta
Zinnia	Zinnia elegans

The following is based on information taken directly from the report published in Cancer. The most important elements of this report are covered in the conclusions as stated in the abstract.

"The Panel concluded that it was not aware of any definitive evidence to suggest that synthetic pesticides contribute significantly to overall cancer mortality. Panel ... did not believe that any increased intake of pesticide residues associated with increased intake of fruits and vegetables poses any increased cancer risk. The Panel further concluded ... tobacco use continues to be the most important preventable cause of cancer and premature mortality and thus is an appropriate focus for cancer control strategy."

The report contains a very well written and concise review of many studies which relate to pesticide toxicology and human effects. During the review of data, the panel also stressed and took into consideration potential sensitive populations, especially infants and children.

A brief summary of their conclusions and recommendations is included, and perhaps the most important point which was stressed by the Panel is the fact that this is a dynamic issue, which must be continuously re-evaluated on the basis of new information as it becomes available.

There were 14 listed conclusions in the study, which are summarized below:

• There are no new data which suggest that the 1981 assessment, which stated that synthetic chemicals are responsible for only a very small percentage of cancers (2%), should be changed; however, more research is needed to help understand the issue of complex, multiple-chemical exposures.

• Published evidence is suggestive that phenoxy herbicides may be associated with Non-Hodgkins Lymphoma after high occupational exposures; however, there is no evidence of risk for the general population; however, there have been no studies specifically designed to measure such exposure to herbicides.

• The Panel supports further studies to explore any links between organo-chlorines and breast cancer.

• Tobacco use causes 30% of all cancers (as well as other adverse health effects) in Canada and is the most important preventable cause of cancer.

• Diets rich in fruits and vegetables are important in reducing cancer incidence at multiple sites.

• Any increase in risk from increased pesticide residue consumption on fruits and vegetables is far outweighed by the protective benefits of increased consumption.

• Use of pesticides to decrease food costs and increase food quality, thereby promoting increased consumption of fruits and vegetables is recognized as important in reducing cancer risk.

• The Panel found no evidence that home lawn and garden use of pesticides are likely to be major causes of cancer.

• Public exposure to pesticide residues is minimal and well below those levels deemed safe by regulatory agencies.

• The Panel supported the re-review of older pesticides to insure that adequate and new data are considered.

• The Panel recognizes that there are few data available regarding pesticide exposure and cancer other than the phenoxy herbicides and supports further research.

• This issue must be revisited and reviewed and amended as newer data become available.

• There was no evidence found to support changing the focus on elimination of tobacco use as the most appropriate cancer control strategy.

• Improving risk communication with the public is encouraged.

The following article presents data from research in citrus done in Yuma, AZ. Soils on the Yuma Mesa are very similar to the sandy textured soils found in many upland areas here on the Central Coast. Some of the practices presented here may be worthy of consideration by Central Coast citrus growers.

For the last three years this question has been investigated by Drs. Glenn Wright and Bill McCloskey of the University of Arizona. Since 1994, the yield and fruit size distribution of young (approx. 7 yrs. at start) lemon trees, located in a commercial orchard on a Yuma Mesa ranch, have been carefully recorded. Comparisons were made between trees having clean culture, with all weed control accomplished with pre- and post-emergence herbicides**, compared to conventional disking and mechanical mowing. In addition, a fourth treatment involved low herbicide rates attempting to "chemically mow" the weeds. This latter treatment was suspended in 1995 due to poor control and replaced with a combination clean culture/disking treatment for the 1996/97 season. This treatment controlled all weeds chemically to remove direct weed competition from the trees, and disking was then done to measure any effect of disking on tree performance.

The total fruit harvested for each treatment for each season is presented below in units of 60# field boxes per acre. The data have been statistically analyzed by analysis of variance and Duncan’s Multiple Range test, which will indicate which mean values are statistically different from the others at the 95% probability range.

Table 1. Yield Data - 1994 through 1996/97
60# Field Boxes/A.

Treatment	1994/95	1995/96	1996/97
Clean Culture	464a	807a	270a
Chem.Mow/Clean and Disk	383(-17.5%)a	--	239(-11.5%)ab
Disk	402(-13.4%)a	697(-13.6%)b	232(-14.1%)b
Mow	425(-8.4%)a	650(-19.5%)b	207(-22.3%)b

Values within a column followed by the same letter are not significantly different, P=0.05.

For three years, the yield of trees under a conventional disking program was depressed by approximately 14% compared to clean culture. This response has been statistically significant (P=0.05) for the last two years.

The data for 1996/97 give some insight into the effect of both weed competition and the direct effect of disking on yield. Trees under clean culture followed by disking show a statistically non-significant (p=0.05) yield response of -11.5% compared to clean culture alone. Disking alone produces a statistically significant -14.1% yield response compared to clean culture. This suggests that a large part of the yield reduction associated with disking may be attributed to the direct effect of the disking operation itself, and part due to weed competition.

Table 2 - Gross Returns Based on Data from Table 1

Treatment	1994/95	1995/96	1996/97
Clean Culture	$2,077	$3,613	$1,209
Chem.Mow/Clean and Disk	$1,715 (-$362)	--	$1,071 (-$318)
Disk	$1,800 (-$277)	$3,121 (-$492)	$1,039 (-$170)
Mow	$1,903 (-$174)	$2,910 (-$703)	$927 (-$282)

The yield data reported in Table 2 are gross returns in $/acre based on a 70% packout and $4.05* per 38# carton.

Table 2 illustrates the value of the extra fruit produced under clean culture compared to disking in lemons over three years. The three-year average is $313 per acre per year reduction in gross returns under disking compared to clean culture, and $386 per acre per year reduction for mowing.

In addition to total fruit production, the percentage of each fruit size as a proportion of the total fruit packed was recorded. The October harvest data for years 1995/96 and 1996/97 are presented in Table 3.

Table 3. Fruit Sizes 115 and Larger, % of Total Fruit Packed

	Clean	Clean and Disk	Disk	Mow
1995/96 Oct. 7 Harvest	19.88a		13.55b	11.37b
1995/96 Dec. 12 Harvest	22.77ab		30.27a	10.08b
1996/97 Oct. 3 Harvest	15.32b	17.43ab	17.05ab	21.60a
1996/97 Dec. 6 Harvest	35.85a	28.92b	29.22b	24.30b

Values within a column followed by the same letter are not significantly different, P=0.05.

The data in Table 3 have been statistically evaluated with analysis of variance and Duncan’s Multiple Range test, at P=0.05. Values on the same line with differing letters following them are significantly different at P=0.05. Fruit size data for 1994/95 are not available. The 1996/97 early harvest was exceptionally light and may not be representative of typical years.

The data in Table 3 indicate that with the exception of the October 1996 harvest, trees under clean culture have produced more fruit of the larger sizes than trees under disking or mowing practices. This effect is generally statistically significant (P=0.05) and should represent an increased return from trees under clean culture since larger fruit sizes, especially earlier in the harvest season, are desirable and priced accordingly. The lack of a significant response in the October 1996 harvest may be related to the exceptionally light early harvest.

The cost of maintaining an orchard under clean culture rather than disking or mowing will obviously depend upon the choice of herbicides, weed pressure, maturity of the orchard, etc. It will probably be more expensive, at least for the first two to three years until the weeds have been cleaned up, to maintain clean culture rather than disking or mowing, but given the yield and fruit size responses measured in this experiment, the increased cost will be easily offset by increased returns. In addition, growers under clean culture report a reduction in water use and faster irrigations and less tree damage from heavy equipment.

Information source: Yuma Co. Cooperative Extension Newsletter. Dec. 1997. Chris Sumner, Research Specialist.

* Yuma County Arizona Citrus Budgets, 5-yr. average, 1988-1993,.
** The herbicide program selected for this project includes 2.5 lb. Solicam + 3 qt. Surflan/ac applied in November, with 2 applications of 2% Roundup during the summer. Spot treatment with 1.5% Poast + Oil was made as needed. See U of A 1995 Citrus Research Report, p.16, for details.