Plant Pathology Fact Sheet PP-33

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Downy Mildew of Crucifers

Tom Kucharek, Professor and Extension Plant Pathologist, Plant PathologyDepartment, University of Florida, Gainesville 32611. 1985, Copied January2000

Cause

Downy mildew of cruciferous crops iscaused by the fungus Peronospora parasitica. Cab-bage, Chinese cabbage (including NAPA andbok choy), broccoli, cauliflower, radish, turnips,Brussels sprouts, mustard, collard, rutabagas,kohlrabi, rape, forage kales, and kale are sus-ceptible. The susceptibility of many crucifer-ous weeds in Florida is unknown; they are in-fected by this fungus but it remains unknownat this time if the strains infecting such weedswill also infect cruciferous crops. For practicalpurposes it is presumed that at least some cru-ciferous weed strains of this fungus infect cru-ciferous crops. Varieties within cruciferouscrops and the crops themselves will vary intheir susceptibility to downy mildew, but in-formation on this aspect is scant. Therefore, donot make hasty decisions about changing vari-eties without first inquiring into the availabil-ity of current information about varieties or firstexperimenting with small plantings of new va-rieties. If varietal reaction to downy mildew isavailable, it would be available for major cru-ciferous crops such as cabbage, broccoli, cauli-flower and maybe radish. On the other crucif-erous crops, personal experience may have tobe relied upon. This issue is further compli-cated by the presence of different strains (races)of this pathogen. The presence of different racesis known, but practically speaking this infor-

mation is of minimal value at this time. Whenrace surveys are conducted and crop varietiesare evaluated extensively each year, suchknowledge could become useful. There is someevidence that strains that infect radish are notaggressive on other cruciferous crops. Also, theweed, shepards purse, has not been success-fully infected with the downy mildew fungususing spores from commercial crops eventhough this fungus was first described onshepards purse in 1796.

The fungus causing downy mildew oncrucifers produces two spore types. One sporeis called an oospore and presumably functionsas a survival spore. It is formed within infectedhost tissue primarily during crop senescence.Then, when the next cruciferous crop is plantedit might serve as a new source of inoculum bygerminating, developing microscopic threads(hyphae) that later produce the second sporetype called conidia. Some reports suggest thepossibility that oospores can be carried on seedif infected plants are used for seed production.

It is questionable at this time whether theoospore stage of the life cycle is of any practi-cal consequence for carry over of this fungus.First, germination of oospores has not been re-ported. Second, cruciferous crops or volunteersinfected with downy mildew abound through-out the year in Florida. During the summer thisdisease spreads slowly but it can exist in pre-viously infected plants either in a systemic or

locally infected manner. Then, when cooler tem-peratures occur, the fungus will produceconidia that are disseminated by wind or winddriven rains. Third, cruciferous weeds may har-bor the fungus in a similar manner as crucifer-ous crops but at this time it is unknown if suchweeds act as reservoirs for strains of this fun-gus that infect cruciferous crops. Some scien-tists believe that weeds are not a source of in-oculum while others are in doubt. Whatever thesource of inoculum may be, it is ample becausewhen environmental conditions are conducivefor downy mildew, this disease can abound oncruciferous crops.

Cool and wet conditions are conducivefor downy mildew of crucifers. Spore produc-tion is greatest from 53° to 61°F but occurs tosome extent from 39° to 85°F and maybe atslightly higher temperatures. When spores arein contact with a leaf with moisture they germi-nate and the germ tube apparatus penetratesthe tissue. Temperatures between 42° to 61°Fare ideal for these activities but they can occurbetween 39°F to 75°F. Symptoms can occurwithin 3 to 4 days after infection if temperaturesnear 75°F prevail during days following theinfection period. As temperatures deviate frompreviously mentioned optima, disease devel-opment and spread is progressively slower.

Moisture is required for disease devel-opment. It is probably correct to say that thegreater the rainfall or overhead irrigation or thelonger the wetting periods from rainfall, dew,etc., the more severe this disease will be if tem-peratures are suitable.

Symptoms

All plant parts of crucifers can becomeinfected with this fungus. In Florida, leaf symp-toms are commonly observed. Black or darkspecks appear on young leaves, usually on theunderside of the leaf first. Such spots are oftenirregular in shape and may appear net-like(Figs. 1 & 2). The upper side of the leaf will alsodevelop dark spots similar in shape and may

be accompanied by leaf yellowing (Figs. 3 & 4).On older leaves, coalescence of these spots canoccur, resulting in larger areas of the leaf bladehaving large, sunken, paper tan-colored spots(Fig. 5). Leaf yellowing, again, may accompanythese symptoms. Early infection on youngplants can cause stunting.

On the underside of the leaf spots, awhite-grey, downy growth can be observedwith or without the aid of a hand lens, espe-cially when leaves are wet. On mature cabbage,downy mildew can appear as dark sunkenspots on the head or wrapper leaves (Fig. 6).Often infections on the cabbage head will re-sult in a purplish tinge. Such infections predis-pose the plant to soft rot bacteria orSclerotiniose (see PP Fact Sheets 12 and 22),which can further rot tissue in the field or afterharvest.

Cauliflower curds and broccoli headscan become infected with blackened areas onthe outside of the tissue. The infection can be-come systemic and turn inner curd and stemtissue dark. Radish and turnip “roots” can be-come infected by spores that are washed downto the soil from the leaves. Symptoms mightpredominate on the upper part of the root butthe entire root is susceptible. Black spotting ora netted appearance can be observed on theoutside of the root but an internal, firm rot canoccur as well in some situations. Some root dis-tortion could occur, especially, if infection oc-curred early in relation to root swelling. Flow-ers and seed stalks of cruciferous crops, espe-cially mustard, are also infected.

Control

Downy mildew is controlled primarilyby fungicides at the present time. Nonsystemicfungicides should be applied at least weeklybeginning when night time temperatures are inthe range conducive for sporulation and dis-

ease development and when rains or irrigationsare frequent or heavy. Systemic fungicides, asthey are developed, will more than likely re-quire considerably fewer applications butshould be used as a tank mix with non-systemic,protective fungicides. Systemic fungicides havea history of losing their effectiveness because“new” strains, resistant to systemic fungicides,increase in population. The tank mix of sys-temic and non-systemic fungicides maylengthen the effective “lifespan” of these spe-cific, systemic fungicides. For details on cur-rent fungicide usage contact your local countyextension office. Downy mildew favorableweather may occur at seeding or transplantingtime.

Therefore, be prepared to spray even at anearly stage of crop development.

It is customary with many plant diseasesto integrate fungicide spray programs with cul-tural controls and resistant varieties. Withdowny mildew of crucifers, however, manysources of spores may exist. The life cycle ofthis disease is imperfectly understood and re-sistant varieties may be difficult to document,making it difficult to recommend a long list ofdefinitive non-chemical controls. Certainly, thegrower should purchase or produce downymildew-free transplants. Excess plants pro-duced in ground transplant beds should beplowed down, preferably with a mold-boardplow, as soon as successful transplanting iscomplete. Similarly, excess plants produced inother types of transplant operations should bedestroyed as soon as possible. Do not dumpsuch plants in cull piles where they might con-

tinue to grow and serve as sources of inocu-lum for downy mildew and other diseases.When growing plants in doors maximize ven-tilation within the structure. This practice re-duces moisture retention periods on the leaves.Transplants grown indoors under partial shadewill retain moisture for a longer period of timeand temperatures might be cool enough fordowny mildew development even though tem-peratures outside are too high for disease de-velopment. Crop rotation with non-cruciferouscrops might be of some benefit.

Where ground transplant beds are used,rotate sites each year. Transplant productionshould be at least 1/4 mile from productionfields if possible. Volunteer cruciferous plantsshould be eliminated. Cruciferous weedsshould be controlled as best as possible in casethey might serve as a source of spores.

This may be difficult or impractical insome cases such as in ditch banks. Weed con-trol in the vicinity of transplant beds should bemaximized. Direct seeded field plantings havebeen observed to have less downy mildew, pre-sumably because the initial canopy is thin andallows for faster evaporation of leaf moisturecompared to the thick canopy associated withsome transplant beds.

Determine which varieties are more re-sistant to downy mildew by consulting seedcatalogs and local data sources. When experi-menting with new sources of seed, isolate themin transplant production areas and fields to re-duce chances of introducing black rot (See P.P.Fact Sheet No. 13) on your farm.

Figure 1. Downy mildew in cabbage leaf. Figure 2. Downy mildew on underside ofcabbage leaf.

Figure 3. Downy mildew in cabbage leaves. Figure 4. Downy mildew in radish leaves.

Figure 5. Downy mildew in broccoli leaf. Figure 6. Downy mildew in cabbage head.